How to determine application conditions: temperature, humidity, overcoating interval, mixing ratio and thinning
The same can of paint gives two different results in summer and in winter, in fine weather and in rain. This page addresses only three categories of question—whether application is possible, when the next coat may be applied, and how the mixing ratio is calculated—which determine whether the coating is finally accepted or reworked.
Table of contents on this page
- Temperature and humidity(5 items)
- Overcoating and film thickness control(6 items)
- Mixing ratio and thinning(7 items)
- Application method(3 items)
- Others(27 items)
Temperature and humidity
- What is the minimum application temperature for anti-corrosion coatings?
- Conventional epoxy and polyurethane systems generally require an ambient temperature above 5°C and a substrate temperature more than 3°C above the dew point; low-temperature-curing types may go lower. The specific lower limit is subject to the TDS for that model.
- At what humidity must application be stopped?
- Relative humidity should generally not exceed 85%, and the substrate temperature must be more than 3°C above the dew point—otherwise condensation forms on the surface and the coating whitens with poor adhesion. After rain and in the early morning are the periods most likely to exceed limits; measure before starting work.
- Can application be carried out in winter?
- Yes, but a low-temperature-curing product shall be used or heating and insulation provided, and the recoat interval extended. Conventional epoxy cures extremely slowly below 5°C; forced application leaves the film wet for long periods and the film properties are unlikely to meet specification.
- How soon after blasting must the primer be applied?
- As soon as possible; the first primer coat is generally required within 4 h (within 1–2 h in damp environments). Beyond this, rust reversion occurs and the surface must be re-prepared. The window is shorter in high-humidity seasons; rust reversion after overrun must be re-prepared and cannot be painted over directly.
- Can application be carried out in rainy weather?
- Not in the open air. Rain washes away uncured film and humidity exceeds limits; indoor application requires ventilation and dehumidification. Rain damages uncured film; indoor application also requires ventilation and dehumidification.
Overcoating and film thickness control
- What does recoat interval mean?
- The time window between completion of the previous coat and the point at which the next coat may be applied: too short causes lifting or solvent entrapment, too long reduces intercoat adhesion. The window is given in the TDS and depends directly on temperature and the product system; the lower the temperature, the longer the interval.
- What is done when the recoat interval is exceeded?
- Carry out surface preparation as required by the TDS before recoating—usually sanding to profile and removal of surface chalk, with wiping using the matching thinner where necessary; do not apply directly over the surface. After overrun the surface is fully cured and smooth; direct application gives insufficient intercoat adhesion and tends to peel off in sheets.
- Why is sanding required before recoating?
- After the minimum interval the film surface is fully cured and smooth; sanding provides mechanical keying, otherwise intercoat adhesion is insufficient and peeling in sheets occurs readily. Surface cleaning alone without profiling leaves intercoat adhesion insufficient.
- What film thickness can a single coat achieve?
- Conventional single-coat DFT for epoxies is 60–100µm; exceeding the recommended upper limit causes sagging, pinholes and solvent entrapment. Total film thickness is achieved by additional coats, not by building up a single coat, otherwise curing is inconsistent through the film.Related models:jotun-barrier-75-cn
- How are WFT and DFT converted?
- DFT ≈ WFT × volume solids. For example, for a product with 66% volume solids, a WFT of 150µm corresponds to a DFT of approximately 99µm. Volume solids are given in the TDS.Related models:jotun-barrier-75-cn
- How is the quantity calculated from the spreading rate?
- Theoretical spreading rate (m²/L) = volume solids × 10 ÷ DFT (µm). Actual quantity is multiplied by the loss factor (typically 1.3–1.6 for spray). The loss factor depends on the application method and is higher for spray than for brush; for large areas, a trial area measured on site gives greater accuracy.
Mixing ratio and thinning
- Which thinner is used for epoxy zinc-rich primer?
- Use the epoxy thinner matched to that model; polyurethane thinner or banana oil shall not be substituted. The specific product is given in the Thinner column of the TDS. The thinning ratio follows the TDS, usually not more than 5%; over-thinning reduces both film thickness and corrosion protection.
- Which thinner is used for polyurethane topcoat?
- Use the dedicated polyurethane thinner. An incorrect thinner causes non-drying, whitening or a soft film. Substituting banana oil or a thinner for another system means the entire topcoat layer must be reworked. Only a dedicated polyurethane thinner matched to the curing agent gives full crosslinking; the rework cost of a substitute far exceeds the small saving.
- What is the mixing ratio of polyurethane topcoat to curing agent?
- It differs by model; mixing must follow the volume or weight ratio in the TDS for that model (commonly 4:1, 5:1 and similar) and shall not be estimated by experience. An incorrect ratio does not dry faster but makes the film brittle and reduces adhesion.
- How much thinner may be added to a two-component paint?
- Usually not more than 5%; beyond this the formulation is effectively changed, affecting film thickness, drying and corrosion protection. Beyond this the formulation is changed; in hot weather even slightly more thinning markedly reduces single-coat film thickness.
- How long after mixing must the product be used?
- Refer to the pot life/usable time in the TDS; it shortens markedly at high summer temperatures. Do not mix more than required at one time. The higher the temperature, the shorter the pot life; in summer, smaller batches mixed more frequently are preferable. Thickening or heat generation in the remaining material indicates the reaction has started; thinning cannot restore it at this point and the material must be discarded.
- Are thinners toxic? What precautions apply during application?
- Most are organic solvents and flammable; provide ventilation, prevent static build-up, keep away from ignition sources and wear a respirator. Confined spaces require forced ventilation and gas testing. Confined spaces (tanks, void spaces) require forced ventilation and gas testing before entry.
- Does adding extra curing agent make the paint dry faster?
- It does not improve. The ratio is stoichiometric; excess curing agent makes the film brittle and reduces adhesion, and the remaining components react incompletely. Mix according to the TDS. The ratio is stoichiometric; excess curing agent makes the film brittle, reduces adhesion and leaves the remaining components incompletely reacted.
Application method
- How is the choice made between spray, brush and roller?
- Airless spray gives the highest efficiency and uniform film thickness over large areas; brush is used for touch-up at edges and welds; roller suits small areas and maintenance. Maintenance touch-up mostly uses brush and roller; welds and edges require a stripe coat before large-area spraying.
- How are welds and edges treated?
- Welds shall be deslagged and ground smooth, and edges given a stripe coat first; otherwise film thickness at these locations is inherently low and they are the first to rust. Film thickness at these locations is inherently low and they are the first to rust; stripe coat first, then spray the entire area.
- What qualifications are required for anti-corrosion coating application?
- Projects subject to formal inspection generally require the applicator to hold the corresponding qualifications and certified personnel, with the material supplier providing the product certificate of conformity and TDS. Qualification requirements are subject to the tender documents and local regulations.
Others
- Why is a power plant coating contract split into several lots?
- Splitting clarifies responsibility: scaffolding and insulation are temporary works, coating is the surface layer, and if combined in one package responsibility for problems cannot be clearly attributed. Such contracts are reviewed first by which outage window they are tied to, then by qualification and track-record thresholds, and only lastly by amount.
- Can new paint be applied directly over an old coating?
- Not directly; three checks are made first: a solvent wipe to see whether the old coating softens, a pull-off test to see whether failure occurs between old and new coats or between the coating and the steel, and probing of blisters to see whether rust lies beneath. Only when all three pass may the coating be retained for overcoating; if one fails, a downgraded treatment is required.
- In a project where the client supplies the paint, who calculates and who approves the stock quantity?
- The applicator calculates and the client confirms in writing, and the purchase documents state clearly the ownership of excess consumption and surplus material. Verbal agreement does not count: whether the schedule may be extended when material does not arrive and work stops shall also be written in, otherwise the difference is borne by the party concerned.
- For painting area on steel structures, what is the difference between developed area and projected area?
- Painting is measured by developed area, not projected area. The surface of a section must be fully unfolded; the developed perimeter of an H-section is often three to four times its silhouette width, and pipe is calculated as outside diameter multiplied by pi. Stocking material by projected area will certainly run short mid-project.
- What loss factor is generally used for paint?
- It is taken by location, not as a single figure for the whole contract. Flat surfaces with mechanical spray: 1.1; general structures with section supports: 1.3; lattice work, railings and platforms with many edges: 1.4. Within one quotation, do not mix in the percentage-loss-rate method.
- How far above the dew point must the steel plate temperature be before painting?
- At least 3°C, in accordance with GB/T 18570.4—2001 clause 3.2. At 80% humidity the margin is only 0.4°C, at 85% it is 2.5°C, and near 92% only 1.3°C. An infrared thermometer shall not be used to measure steel plate surface temperature; a contact-type instrument is required.
- At a steel temperature of 5°C with epoxy, how long before the next coat can be applied?
- First check the recoat table in the TDS for that paint: one low-temperature epoxy gives 12 h to 8 weeks at the 5°C level. Hard dry time has a separate column and is often later than the lower recoat limit; the programme shall be scheduled to the later of the two.
- Must hot-dip galvanized parts be rust-removed to Sa2.5 before painting?
- No. Galvanized surfaces are treated by light sweep blasting: fine abrasive at low pressure to remove the passivation film and zinc ash and to create slight roughness. Blasting to Sa2.5 would penetrate the zinc layer and expose the steel substrate, and corrosion would then start first at the zinc-to-steel boundary.
- Blasting abrasive has caked from moisture; can it be used after drying in the sun?
- Not recommended. Caking indicates water has been absorbed; spreading it out to dry dries only the surface while the interior remains wet. Inside a closed tank this transports the water in with it and voids the dew point margin measured before starting work. Squeeze a handful: if lumps or damp marks are present, replacing the entire batch is simpler.
- When should recycled abrasive be replaced with new material?
- Judge by the fines content, not by the number of cycles. Repeated recycling makes the particles finer and increases dust; heavy dust covers the freshly prepared steel surface so that inspection cannot distinguish true condition from false, and anchor profile depth is insufficient. When the screened fines increase markedly, add new material or replace the entire batch.
- For an annual framework coating contract, should standby costs be included in the quotation?
- Yes. The framework does not commit an annual volume, yet crews still require wages, certificates and equipment in months without work orders, and this cost can only be spread into the unit rate. Pricing on area alone will not last to mid-year.
- The pipe is already installed; can the coating be repaired at the support plate contact surface?
- Repair is possible, but the method differs from new construction. Lifting the pipe for repainting is the most reliable; if lifting is not possible, remove loose rust and salts, penetrate the joint with low-viscosity epoxy, then add sealing or a backing layer per the specification. After repair, retain photographs and DFT readings.
- Should support painting be carried out before or after pipe erection?
- Before pipe erection; this is the main process. Remove rust to Sa2.5 or St3, treat welds and edges to P grade, and complete primer and intermediate coat before pipe erection; touch-up after pipe erection is remedial only, and if the recoat interval is exceeded, abrasive sweep is required first.
- If abrasive has absorbed moisture and caked, can it be used after screening?
- Not recommended. Caking indicates that the batch has absorbed water; introducing it into the tank carries moisture into the enclosed space, and the blasted steel surface will flash-rust faster. If a handful can be squeezed into a ball and shows wet marks, replace the entire batch; the cost of replacing abrasive is not on the same order as reworking an entire tank wall section.
- Recycled abrasive becomes finer with use; can longer blasting compensate?
- Cannot compensate. As particles become finer, the dust fraction increases, the blasted surface turns grey, and roughness cannot be achieved; extending the time only repeats the blasting pass. The remedy is to add new grit and adjust the steel shot-to-steel grit ratio; when fineness reaches a certain level, replace the entire batch.
- Can a single composite unit rate be quoted for internal and external tank corrosion protection?
- Not recommended. Scaffolding, ventilation, and labor efficiency differ between the two sides, and the schedule often starts with the exterior and then the tank interior. Itemize prices by area and process; this aligns with the schedule and reduces later disputes over idle labor.
- Why does the number of blasting booths and painting booths differ by more than three times?
- Because the two occupy workstations differently. Blasting enters a section once and exits; painting requires entry by coat number, with waiting for recoat between coats, plus color-change cleaning, so occupancy time is roughly three times that of blasting. Hence two blasting booths to seven painting booths is common.
- How many coats are required for a total DFT of 300 µm on tank interior walls?
- Four coats: two primer and two topcoat. Control DFT per coat at 75–90 µm; four coats are needed to reach 300 µm. For a system that can achieve only 75 µm per coat, four coats just meet the limit; control actual spraying at around 80 µm. Reducing one coat requires 100 µm per coat, which tends to cause sagging and solvent retention on vertical surfaces; blistering and pinholes may not appear until one or two months later.
- What recoat interval is required between the four coats inside the tank?
- Determine by substrate temperature, not by clock time. The recoat interval is a pair of values: at 15 °C substrate temperature, the minimum is about 4 h and the maximum several weeks; at 5 °C the minimum is 12 h, and hard dry time may exceed the minimum recoat interval. Calculate a window for each coat on the day and record it in the construction log; if the maximum is exceeded, treat the surface as an aged coating for recoating.
- Can fireproofing coating and anticorrosive paint be applied on the same day in succession?
- No. The primer must be fully cured, not merely touch dry; use the curing time for the corresponding substrate temperature in the data sheet. Applying intumescent fireproofing coating at an insufficient interval will not produce blisters on the same day; by the time they appear, large areas have already delaminated.
- The paint is no longer tacky to touch; can the second coat be applied directly?
- No; that is only touch dry. Recoatability depends on through-dry condition and the recoat interval given in the data sheet. Assess on a test panel under the same conditions; do not perform destructive checks on the workpiece, as the tested area would require touch-up.
- For winter application, which temperature should be used to calculate curing time?
- Use substrate temperature, not air temperature. The fully cured time in the data sheet is referenced to about 25 °C; at a steel plate temperature of 10 °C, it is much longer. Below the lower limit specified in the data sheet, curing may stop; only wait for warming or apply heat.
- Should flange bolts and nuts be painted?
- Do not paint threads or nut faces; coat the exposed hexagon of bolt heads and nuts fully. Paint on threads distorts torque readings and causes thread slipping during disassembly; water accumulates on exposed hexagons and initiates rust, which is the corrosion starting point.
- Which areas require masking during painting?
- Sealing faces, threads and nut faces, valve stems, instrument connections, nameplates, and grounding tabs. Use PTFE sheet or dedicated caps for masking; wrap threads with tape, not masking tape that leaves adhesive residue. Remove masking after the paint has hardened; feather hard edges with abrasive paper after removal.
- What overlap is required between heat-shrink sleeve and pipe body coating?
- Greater than 100 mm is the lower limit, not the target. When measuring, take the minimum of eight points around the circumference, not the average; the sleeve shrinks unevenly in thickness, and the average will conceal the thinnest point. At the overlap, grind the old pipe coating to a taper, leaving no hard edge.
- What length at the pipe end should be left uncoated?
- Generally leave 100–150 mm; the specific figure is fixed by the specification. This length is the interface reserved for field joint coating: the coating yard should not paint it fully, and the site should not cover it directly with topcoat. If covered, thickness and adhesion will be incorrect, and holiday testing will show defects at every coat.
- Can field joint coating be performed in high humidity or rain?
- Check dew point and humidity: stop when humidity reaches 85% or above, or when the steel surface temperature is not more than 3 °C above the dew point. Adhesion is poor under these conditions, and pull-off testing will expose the substrate. If the schedule is tight, use a rain shelter to control the local environment; do not gamble on the weather.
The three places most likely to go wrong on site
- Mixing hardener by feel, ignoring whether the ratio is by volume or by weight.
- Applying the next coat directly after the recoat interval has expired, without abrading.
- Forcing standard epoxy in winter; when it does not dry for days, blaming the paint.
Not sure how to specify your application? Send us theArea, substrate, corrosive environment, design film thicknessSend us the four items, and we will provide system recommendations and quantity calculation based on the manufacturer's TDS.
Other questions:1. Selection and systems 3. Inspection and acceptance 4. Defects and repair