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Ask the Paint Chemist – trick for making paint dry

Listed under Blog, newlife paint Posted Jan 07 2015

newlife paint chemistAsk the paint chemist Keith Harrison. Keith is a real paint chemist at Newlife Paints. He collects un-used water-based paints and using his knowledge, produces a range of effective and cost effective ceiling and wall paints and eggshells. I used the ceiling paint, white over magnolia on a stippled ceiling. I achieved one coat coverage.

Luckily, Keith still remembers how solvent-based products work and he answered this question from a reader. (In essence, how was the reader able to make a non-drying oil-based paint dry by applying a coat of heavily diluted oil paint over the top!)

A long time ago (40 yrs.) I varnished a floor over the old varnish and failed to adequately remove the floor wax. After waiting a month for it to dry, a old painter told me to redo the floor with a 50/50 mix of the same varnish and mineral spirits. It worked. Had a similar problem recently using what must have been old oil based exterior primer. Used the same trick with another primer and it worked again. Can you tell me why?

Steve

Keith answers as follows:

An interesting question.

My comments, providing I understand correctly.

… this is a bit guess work, without seeing what was done, knowing which coatings were diluted and which mineral spirit was used, but I’ve assumed white spirit.

With old coatings, particularly those containing wax or a full wax coat, a good quantity of the old wax will get dissolved into the new applied coating. When such old waxes get to a reasonable level, when they come in contact with new varnish, they both soften and delay the curing of varnish, potentially leaving it soft and sticky for weeks or maybe almost permanently.

(i.e. if you have a build up of wax, a conventional overcoat of varnish will soften enough wax to cause a very long / indefinite slow drying process.)

If you apply a 50/50 mix of new varnish/mineral spirit, there are two possible reasons why this “drying trick” may have worked.

– Firstly, once the white spirit evaporates off, there is only half the amount of new coating present, but you may well have applied enough new “oxidiser” into the varnish coating below to overcome the effect of the wax.

– If you used a 50/50 blend in the first place, this is quite a low actual level of new coating. If you apply quite a thin final coat, it is likely to evaporate quickly, leaving a thin coat which may prevent the wax/old coat to get sufficiently entrained (dissolved) to cause an issue with the new coat.

Hope these thoughts may shed a bit of light?

————————–

So there you go. Thanks Keith for taking the time out to explain what may have been going on.

Hot coating varnish

There is a similar counter-intuitive and deliberate! trick with solvent-based varnish, called hot coating, i.e. apply first thinned coat of varnish to bare timber, and then as soon as it has tacked off, go straight over with a second coat.

The text books say you need the previous coat to be thoroughly dry before overcoating, but as long as the first coat has skinned over and you don’t disturb it with too forceful a brush action, the second coat will lay on just fine and the two layers can then dry and cure almost in unison.

When dry, continue as normal, with almost full strength varnish, one coat per day, to build up the layers. If working on exteriors / boats, it is best to leave denibbing between coats till the final few coats of yacht varnish, so as to not reduce the overall thickness and maximise the coating’s anti-UV capabilities.

Any other strange paint-drying experiences?



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