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Paint being mixed in a workshop
Shop tips · 8 min read

Mixing catalysts, pot life and why you do not thin first

Most failed catalysed jobs are mixed wrong, thinned too early, or sprayed after the pot life. The tin already told you the ratio. The clock starts when the catalyst goes in.

Paint being mixed in a workshop
Paint being mixed in a workshop
The clock starts when the catalyst goes in — not when you remember to thin.
The clock starts when the catalyst goes in — not when you remember to thin.
Measure the ratio on the product you are actually spraying. A PU hardener is not a generic splash.
Measure the ratio on the product you are actually spraying. A PU hardener is not a generic splash.

Two-pack lacquer is not difficult. It is unforgiving. The mix ratio on an acid-cured topcoat or a PU hardener is a recipe, not a hint.

Read the ratio on the product you are actually spraying

A typical Novalk acid-cured clear, such as AC Clear Top Coat, mixes 10:1 with NCH999. Polyurethane systems use their own hardener — and a Sirca PU hardener is not a substitute because the tin is the same colour. If you are on a Sirca PU converter and low monomer hardener, stay inside that system.

Measure. Do not pour “about a splash”. A kitchen plant that scales by weight gets fewer surprises than a shop that scales by eye.

Catalyst in, then wait, then thin if the gun needs it

The sequence we see work in Irish booths:

  1. Stir the base until it is uniform — pigmented product especially.
  2. Add catalyst or hardener at the stated ratio.
  3. Mix thoroughly, scrape the sides.
  4. Observe the induction time if the data sheet asks for one.
  5. Add thinners only when you are spraying, and only the thinner named for that chemistry.

Thinning before catalysis changes the ratio in the pot. You think you have 10:1. You have something else, plus a shorter or dirtier pot life.

AC systems often have a long pot life if you leave them alone — on some Novalk AC clears, 48 hours without AC/PC thinners, shorter once reduced, longer with a specified retarder. That is not permission to mix on Monday for Friday. Label the pot with time and ratio.

When the mix has gone

If the material thickens, gels, or sprays with a dry, dusty feel that no gun tweak fixes, stop. Do not “save it” with more thinner. That is how you get a film that never hardens properly and a callback six weeks later.

Dispose of catalysed waste as the SDS directs. Do not pour it back into the bulk tin.

Isocyanates are a training issue, not a mixing trick

PU hardeners that contain diisocyanates sit under REACH training rules. If your staff spray them, they need the documented course — we explain that in diisocyanate training. Mixing PPE and booth extraction are part of the same discipline as the ratio.

Ask the counter before you invent a system

If a stain, insulator and topcoat came from three conversations, they may not belong in one stack. Call sales with the substrate and the sheen. We would rather specify a full polyurethane or acid-cured system than watch a homemade sandwich fail.

Apply this on a job

Tell sales the system, sheen, colour and when you need it on the gun.

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