Acrylics over Poly urethanes

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PostPosted: Wed Aug 28, 2019 7:53 am
Folks,

Any issues with putting acrylic clear over 2 pack paints or urethanes?

Not car related, guitar related :), red scuff pad sufficient enough for keying

Thanks

Steve



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PostPosted: Wed Aug 28, 2019 9:48 am
i'm not sure what you mean by acrylic, are we talking lacquer? anyway just about anything will go over catalyzed urethanes, note I said just about. there is always the time when certain products don't like one another. lacquer is especially nasty. i'm wondering why your not using a urethane clear? and yes a scuff pad should be ok I like to sand with 600 -800 just to remove any nibs that might have gotten into the color the scuff pad won't do this.
Jay D.
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PostPosted: Wed Aug 28, 2019 5:02 pm
Jay, what you call lacquer is called acrylic here in the land of football, meat pies, kangaroos and Holden cars.

Simso (who is obviously an Aussie because his name is shortened to end with an "o"), yeah, all good mate, only risk is where you have a rub through, exposing a feathered edge which sometimes can be a bit sensitive and fry up. Otherwise, as Jay says, although I'd use grey scotch rather than red, which can leave fairly deep scratches that might be visible through the clear.
Chris



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PostPosted: Wed Aug 28, 2019 6:43 pm
Haha, yes an aussie down under for sure.

That was exactly my concern, i noticed on a feathered edge the poly coat lifting under the acrylic (lacquer coat), made no sense to me as the poly is chemically hardened and sanded, so why he heck is it lifting :),

definetly aware of painting over enamels, but first time i had come across this situation where the feathered edge kept lifting, i seem to have resolved it, by lighter coats and multiples to close that edge over

Steve



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PostPosted: Wed Aug 28, 2019 10:37 pm
HaHa , I knew that post was going to be a problem. its confusing sometimes. i'll watch for the names ending with an O from now on.
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PostPosted: Wed Aug 28, 2019 11:31 pm
badsix wrote:its confusing sometimes.


:happy: Nah, it's easy. Shorten most words to first syllable and add "o". Exceptions are footy (football) and cricket (which is sacred and never shortened). Anyway, only important ones you need to know are servo and bottle-o. :wink:
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PostPosted: Thu Aug 29, 2019 6:35 am
Whats a good drying time before flat sanding and buffing?

Steve

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PostPosted: Thu Aug 29, 2019 7:55 am
24 hours at 20C is the recommended time. I've buffed after 4 hours but need to make sure each coat has flashed well.

A lot depends on the thinner you use, too. With acrylic (clear or solid colour) I only ever use Dulon AAA. You can use a multi thinner with basecoats since it flashes faster but no good for topcoats where the slower flash of the AAA means better gloss, right off the gun.

I only use Axalta 380S acrylic clear. With AAA thinner and a splash of retarder if it's hot - goes on almost like a urethane clear.
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PostPosted: Thu Aug 29, 2019 12:59 pm
Just out of curiosity,

What is a 2-pak basecoat? Is that a hardened Single Stage polyurethane? Or a basecoat with hardener added? or...?

Why use a lacquer clearcoat?


Like I said, for me it is just curiosity as I live in California, where soon we will only be painting with filtered all-organic hemp-based non-GMO air based products with huge "fees" tacked on, while you guys live in the land of one of the two fearsome Rugby players (the other being Samoa!)
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PostPosted: Thu Aug 29, 2019 3:00 pm
OldFatBald wrote:What is a 2-pak basecoat?


Generally basecoats are not hardened. In solvent they're usually modified acrylics but, in some cases a small amount of hardener (5%) can be added to increase stone chip resistance.

Some people refer to more modern systems as two pack basecoats because they were developed in conjunction with two pack clears. The reality is that you can use urethane clear over any base (straight acrylic or modified). The later systems use a reducer which is more compatible with urethane but I've put acrylic clear over them with no problem - just have to make sure that they're really well flashed off or you'll get a reaction or solvent boil.

Solid colour acrylics can be single stage in the sense that no clear is required over the top. Most acrylic that are metallic or pearl do need a clear coat but back in the 1970's there were single stage acrylic metallics.

Obviously water borne is different.

OldFatBald wrote:Why use a lacquer clearcoat?


Firstly to be period correct. A car that was originally painted in acrylic, or even older technologies, would not be considered properly restored if done in a modern urethane clear.

Many show cars are also done in acrylic. It sits flatter (no peel or urethane wave) and buffs up to a gloss level that can be even better than urethane. Needs a lot more coats though since solids content is much lower and this bumps up the labour component, as does the need to buff to get a really high shine.

There is also the way that it bonds to the basecoat. Being the same, chemically, it actually melts in to the base so you get what is essentially a single coating. That means that you don't get clear coat separation like you do with urethane clears and, given proper care, acrylic can outlast urethane. Without care it does tend to dry and crack/craze but that is usually over a much longer time than current generation factory clears. It's lost favour because urethane clears require much less labour input, essentially off the gun can be good enough.

Local laws can be an issue, too. For example, in the UK (and some of Europe) using a paint that has isocyanates is highly illegal except if done in a proper booth. Since they're avid DIYers there, they can only use acrylic in a home situation. That's different to the US (and Canada?) where the VOC (quantity of organic vapours) given off is the mandated limitation and was the part of the reason for development of HVLP guns which increased the amount of paint that went on to a car, rather than was lost in the atmosphere. A bit silly really when the thinner evaporates out of the paint on the car to join what was lost in overspray, anyway. Low VOC systems often use a much lower amount of thinner/reducer so ratios are 4:1 rather than up to 1:1.5 (base:reducer).

We don't have either of those limitations but Worksafe would not take kindly to use of isocyanates out of a booth where there was any possibility of the vapours being breathed in. That said, the mobile guys get away with it out in the car parks, so go figure.

In most of Asia acrylic is still the preferred system since it can be done almost anywhere (no isocyanates) and is much cheaper. Paint companies don't want to sell it though with many having dropped their acrylic line(s). More profit in high cost products.
Chris
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