Finishes, drying and curing

[Brendan]

Music Student - check out the videos from Jarrod under the guides section - http://www.pitbullguitars.com/videos/ - you're looking for Hi Gloss Finishes 1 & 2. Jarrod has built some amazing guitars (kits and scratchies) and his spray finishes are amazing. He ended up building a spray booth using a second hand water tank from memory...

#1 is here:

[Simon Barden]

You'd normally sand after any grain filling and then after sanding sealer coats.

If you're going for a solid colour and using a primer, then you'd spray several primer coats, enough to sand that back flat, then spray your main colour coats. You'd probably sand that flat and then do any clear coats, sand that back and polish once cured.

With a sunburst, you don't want to sand the finish if you can help it at all, so you'd spray all of that, then all the clear coats, before sanding it back and polishing once cured.

What you might do at the beginning of each day's spraying, is do a 'drag sand' - very lightly wiping the surface with some fine sandpaper just to remove any protruding dust particles or hairs in the finish. You need to let the finish dry quite hard before doing this otherwise it's amazing how much (sub-surface) wet finish a dust grain can take with it, so never do this shortly after spraying. Also, if you get an obvious paint run, then you'd probably want to sand down that small area - but again, the finish needs to be dry otherwise you'll break through to the wet layer underneath which is when it all starts to go wrong. Every time you spray, the solvent in the lacquer/paint 'melts' the layer(s) beneath a bit, so there's always a thicker soft layer underneath the apparently dry surface than you'd think.

It takes a while for all the solvent to finally evaporate and the finish fully harden. The harder it is, the better it will polish up. You can sand it down after a few days (the extra surface area provides by all the tiny score lines will help promote solvent evaporation) but ideally wait 3 weeks before the final polish.

[MusicStudent1]

Thanks for the replies. Yes, I watched the videos and that's what gave me the idea for the spray booth. Mine will be simpler but it should keep dust off the drying paint. I also didn't know on a sunburst finish one does not sand between coats.

I intend to do a gloss black nitro finish. I'm just trying to do a good job and avoid burying it in paint as one sometimes sees on inexpensive guitars.

I may use sanding sealer from stew mac followed by a white primer before spraying the first coat of black. I'm still a little confused though. If I spray a coat of black, I need to wait weeks before lite sanding and spraying another coat? That's what it sounds like. Hey, whatever it takes, I'm not going to rush it.

I have some reading to do, thanks for the info.

[Simon Barden]

No, the long wait is only at the end after you've put your clear lacquer coats on.

Normally recommended a maximum of three coats a day, with a 20-60 minute drying time in between each coat. After you've finished the primer coats, I'd wait maybe two days before it's hard enough to sand it down flat down nicely (If it's not hardened enough before you do this, the primmer will still be contracting as it dries further and after a bit more time the the surface won't be flat). Then once flat (you need to look at it with a light shining on it at a low angle to see any small hollows in the paint that still need removing), you can start to spray your top coats on.

If you see any obvious hairs or dust marks stuck to the finish, then do that light drag sand at the beginning of each day before spraying, so that the paint's hardened overnight.

Just FYI, the reason you don't sand a sunburst finish until it's covered in clear lacquer is that the actual fade/burst area is made up of millions of very small lacquer droplets. These will have come from the very edge of the spray, so won't have hit the surface very wet at all. so have very little solvent in them and so don't adhere to the paint below well. If you try and do any sanding on these areas, those small drops come away from the surface very easily, and instead of a 'burst, you are left with clearly defined edges between the different lacquer colours. I found this out the hard way!

[IngridM]

No, the long wait is only at the end after you've put your clear lacquer coats on.

Then once flat (you need to look at it with a light shining on it at a low angle to see any small hollows in the paint that still need removing), you can start to spray your top coats on.

Simon, when spraying top coats what's the procedure and timeline of sanding/respraying?

My stepson has made a Van Halen paint job and is up to the top coats.



I'm presuming it would be good to use the top coats to level out the surface by sanding back between top coats? (Or is this not how top coat works? Total newbie here...!)

The white parts has only 5 total layers, the red 7, the black {not pictured} has 9).

Thanks in advance!

Getting the 14 yo to wait out a 30 day curing time is going to be interesting...

[Rabbit]

Note that Nitro is hazardous to health, and you need to buy a vapour mask and should cover up as much as possible to stop it getting onto your skin - wear rubber gloves or disposable latex or vinyl ones. If you can smell the spray with the mask on, then it's either not seated well enough on your face, or the carbon filter is all used up and it needs replacing. The vapour is also explosive, so use a well ventilated area and no smoking or sparks.

This goes double if you are using automotive 2k clear. You need a full face respirator with carbon filters. A half mask is fine for base, but for clear , the nasties in the hardener can be absorbed through the eyes. Wet sanding can be done after a couple of days, but to fully cure without the aid of baking takes 8 weeks.