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Part 10: Paint Faults (1 of 2)

I’m not being a pessimist ... perhaps “pragmatic perfectionist” is closer to the mark - to my eyes, something has always gone wrong with each of my paintjobs, even the ones I’m proudest of. 

Possible “disasters” include an insect landing, accidently brushing your sleeve on the paint, spraying the wrong paint (e.g. puffing on primer instead of a second coat of colour), dust, runs, weather changes, lumps from the can, etc, etc ...

As you get better at painting, chances are that only one upset will happen per paintjob, and it will be a fairly trivial one. If you ever get to the point where you think you’ve made no errors at all on a paintjob ... well, either you’re being too uncritical, or you’re not stretching yourself: there should always be something you think you could do better next time.

The thing is, most of the time these are not disasters at all, but they do have the potential to get worse if you take the wrong action. That is, generally, anything at all: picking/wiping that insect off, spraying “just a little bit more” to “even out” a run, etc. Anything you do will make it worse.

The “correct” action is to do nothing at all. And keep doing nothing for a week or so.

After that time, the dust or insect that landed on your wet paint can be brushed or rubbed off (& the edges of his little footprints delicately sanded if needed), the run may well have shrunk back to a size where you need do nothing about it (or maybe just a very delicate sanding), the puff of primer you put on could well just need colour coat spraying over it, and so on. If you’d wiped that insect or run off, chances are you would have wiped off a much bigger area than you needed to, probably smearing lightly melted primer with it, and maybe even the plastic underneath. That can be rescued, of course, but it’s much more involved.

Let’s have a look at some common paint faults & what you can do about them:

 

    jr_pf_001_paint_horrendous_bloom_800x600Horrendous bloom

    jr_pf_002_paint_orange_peel_800x600Orange peel

     

    Overspray

    In the 1:1 car spraying area, “overspray” is paint particles drifting from the panel you’re painting to one that you’re not. This paint dust then takes on the look & feel of sand paper when you spray over it. This shouldn’t really be a problem on an RC shell, hopefully you should be able to spray a coat of paint over the whole thing in one hit before it starts drying at the other end.

    Assuming you have got overspray & you’ve spotted it before you’ve painted over it, a very, very light sanding will fix it ... if you have sprayed over it, gentle sanding (just a lot more of it) is still going to be the answer.

    This is probably the reason people think sanding between coats of paint is necessary ... I guess it depends on your technique.

    Bloom

    This is a milkyness/whiteness/dullness that appears on the paint surface, the cause is painting at too low a temperature and/or too high humidity.

    Technically it’s because the solvent in the paint effectively draws any heat in the shell away to make it evaporate, and if the air temperature is already close to dew point (the point at which water will condense out of the air) then you’ll get water forming in & on the surface of the paint.

    Keeping an eye on the humidity and temperature (and not pushing your luck) is the way to avoid this happening in the first place. If it’s already happened, leave the shell for a week & think about how bad it is: it might be that you can get away with just a gentle sanding.

    Check out the first pic for an example of some really horrendous bloom: this is VW “Jazz Blue” metallic paint on the front fender of a Sand Scorcher, caused by me thinking I could get it done & dry before the forecast rain came. This probably could have been salvaged if it wasn’t for the raindrop splats & the other faults caused by trying to rush it.

    Warming the paint up before you use it can also help: if the paint’s warm when you start, then it doesn’t suck quite so much heat out of the shell & the surface won’t get quite so cold. I just stick the can in my back pocket for a while before I use it.

    Uneven/Matt Finish

    Like bloom, but not quite as bad – this is where the condensation has formed a little later in the drying process, and rather than mix with wet paint & form an emulsion (hence the cloudiness), it’s formed right on the surface & just damaged the chances of forming a nice glossy surface. Waiting for a good day & spraying over the top of it, either with more colour or clearcoat should fix it. You do need to be very clear that it’s not bloom: if it is & you spray over it, chances are the water will migrate to the top of the new paint & cause the same problem.

    Orange Peel

    So named because although the paint may be shiny, it’s far from flat: it looks like the surface of an orange. Inadequate solvent in the paint, or inadequate pressure in the can be the cause, but in general it’s overly timid spraying, i.e. not getting enough paint on to properly “wet” & self-level.

    Putting enough paint on for it to get/stay wet enough to level out is the obvious answer ... but if you go too far you’ll have to deal with runs. It’s probably better to put several coats of clearcoat on over the top, and if that still looks like orange peel, either live with it or embark on “rubbing out.

    Look at the nasty green pic for an example of orange peel: this is on the side of my Cheetah project after a couple of light coats of colour.

    Spatter

    Like orange peel, but worse. Clogged nozzles & faulty cans with too little solvent and/or too little pressure are the culprits. Change or clean the nozzle, or return or bin the can. Depending on the severity, spraying over the top to fill in the gaps may fix it, or it’s back to sanding again.

    Runs & Sags

    Also sometimes known as drips, these are caused by putting too much paint on in one go, and/or spraying too close. I’d differentiate between the two: a run usually happens at an edge, either the end/corner of a panel or at a detail line & is typically 4-5mm or so wide & maybe 10-15mm long; a sag on the other hand occurs mid panel and is typically much wider & shallower (maybe 25-40mm by 10mm?).

    Of course, if you really overdo it, a sag can break out into a run as well. As for preventing it ... don’t put so much paint on – resist the temptation to put on “just one more puff”. It’s a balancing act though, go too far the other way & you end up with orange peel.

    If it’s already happened, first do nothing until you’re very, very sure the paint has properly hardened – which will take a while – remember that you’ve put too much paint on, and because it’s so thick, it may well have formed a dry crust on the outside & still be gungy on the inside. A couple of weeks isn’t unreasonable. Don’t test it by digging a thumbnail in it.

    The trick is to remove just the extra paint from the affected area without damaging the rest – easier said than done though. The byword here is patience: aim to do it slowly & don’t be disappointed if it seems you’re not making progress, anything you do that doesn’t wreck it & force you to start the whole thing again is good. I’ve seen various methods expounded: metal masks for shielding & razor blades or power tools to remove the run, for example ... but I prefer a much gentler approach: masking off the areas I want to protect, very gentle grades of wet & dry paper, and small sanding blocks so I’m always presenting a flat face to the run. Although maybe if I got lots & lots of runs I’d find a better & faster way of doing it ;)

    I was going to insert an example here of a run (in clearcoat) on the back of my Bullhead, but by the time I’d left it a week it had shrank to practically nothing, it was in an awkward position, and it would be hidden by the roll bar & flag. in the end I just lived with it, the marginal benefit of sanding it was outweighed by the very real possibility of making it far worse. Doing nothing is a valid strategy sometimes :)

    Next Time ...

    I’ll cover “Mottling”, Foreign Bodies, Sanding Marks, Remelt, Paint Reactions/Pinholes, Bleed & Chips in the next installment :)

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    Written by TB member Jonny Retro