Acrylic or Polyurethane paint? Etc...

[cockney steve]

@ Croqueteer. Domestic oil-paint is the worst of all worlds for painting an aircraft.....If you want an air-drying (as opposed to chemically curing) paint, use Transport polyurethane, AKA "Coach-enamel" warm it to thin it , spray or brush...super gloss, lower shrinkage than dulux, quicker tacking to "dust-dry" quicker hardening , tougher and less shrinkage. any automotive paint supplier will be able to colour-match exactly as well as supplying the correct thinner, primer etc. to give you the best-protecting, most durable finish.

I have ,myself , sprayed domestic radiators with thinned, warm Dulux...excellent finish but a bugger to handle until hardened.

much better to paint the inaccessible portion only, then fit and commission.....spray/roller/brush the acessible bits when it's cooling down....too hot and it will have a dull surface spoiled by "solvent-pop" or a brush will tend to stick.....get it warm enough and all brushmarks will flow out, leave for a couple of hours and then turn up the heat and it will bake itself

[mr_rodge]

cockney_steve,

Thanks for the in-depth reply. I'm using a 2-pack paint with Valspar UD-429 instead of the hardener (same 2:1, but with about 30% thinner), which allows the paint to air dry. It apparently won't kill me, but the smell certainly gives the impression it will so I still wear a respirator & goggles, just in case. From what the guy in shop said it's similar to the stuff they use to make aerosol paints dry. It supposedly sets just as hard, but takes much longer to cure.

Visited a shop yesterday and they gave me some acrylic paint to mix with the Valspar, hopefully it'll work just as well as the last bit I had.

My next conundrum is the spraying... I thought I'd done it pretty well until I saw the orange peel at a certain angle, and I wondered why the 30% gloss paint looked high gloss.

The guy in the shop said I'd probably had the gun too close to the panel and that's why it dried glossy, he said to spray it on 'dry' and it'll have the intended finish. I'm using RAL 9010.

I'm now wondering if it's the cold air causing the orange peel or my crappy gun, if it's the latter can you recommend a compressor/gun rig? If it costs me £250 it'll still be cheaper than me replacing 20-odd cabinet doors. I always keep the paint and panels I'm practicing on in the house and only take them to the cold garage to spray, as soon as they're dry enough to handle they come back in the house.

Thanks again.

[cockney steve]

I'm not familiar with the Valspar, in fact the whole refinishing trade has changed dramatically since the limitations on voc emissions.

However, a broad generalisation or two.

do not mix different manufacturer's products and systems.

just because a 2-pack product will air-dry, doesn't mean it will have the same characteristics as the proper, catalysed chemically-cured mix.

A "dry" coat willtend to show more orange-peel also the film density and strength are questionable.

30% is a LOT of thinner! note the "speed" of a thinner has a huge effect on the finish...you really need to understand the science behind spraying ...there are many textbooks and some manufacturers used to do excellent tech. sheets. / maybe still do.

Viscocity of the medium is affected by temp and solvent-content....you then need a correct air-pressure and volume to atomise to right droplet-size (uniform,with minimal fog or heavy particles) then the thinner-formulation.....once the fluid is atomised, the "fast"portion evaporates between nozzle and work...thus the droplet consolidates...it should contain enough solvent, that it flattens and dissolves into it's neighbouring droplets without splashing a fog everywhere (dulls the finish and alters metallic colours)...too little that flashes -off too quickly, means the droplet won't fully flatten and merge = "orange-peel! "

So, no matter how warm the paint and the work, if the ambient is yoo cold, your finish will be affected.

Another little-understood facet ,=the air-supply!
a compressor and airline act like a primitive fridge!!!!

air is compressed and so is the heat it contains so your compressor gets hot and the moisture condenses out (important to drain the tank regularly and use an oil/water separator on the output)

the tank full of compressed-air will stabilise at ambient but, of course, contain less heat ( think kettle of boiling water V egg cup full....both contain boiling water, the kettle holds a lot more heat )

When you pass it through the spray-gun, it expands...suddenly , the heat in 1 cubic of compressed air,is now dispersed into , say 5 cubc feet of free-air...so the temperature drops your paint-droplets congeal, but don't flash-off the solvents the same as when they're warm,,,also the temp-drop can condense ambient moisture into the paint and onto the painted surface......

I'm only scratching the surface of the subject, but observe the basics...clean, dry and warm, thin, properly "wet" coats (although there are exceptions - some etch-primers are specifically dusted in a dryish translucent film) correct viscocity and correct volume and pressure at the gun.....airline has a known pressure-drop depending on diameter and length, so 60 psi at the regulator ,could well be only 35 at the gun!

Many old-school painters knew nothing about the forgoing, it was all empirical knowledge, passed-down through apprenticeship and experience,-what worked, what didn't and the little dodges....paint too thick? reduce flow-rate /move gun closer to the work, and so-on.

hth. steve

good results can be obtained with very modest gear....many young mechanics are conned into a chestful of snap-on spanners at £20 a go and many painters are convinced that ONLY a£200 De Vilbiss gun will paint properly......in both cases, it's a mix of brain and brawn....think objectively and apply to your hands...you can't buy skill but you can readily learn it!