Polyurethane wearing quickly

[kman42]

Hi, I've read great things about the no-fuss durability of spray-on polyurethane clear coats, so I gave it a go on one of my guitars. This was a white strat with a plywood body, which had bog, rattle can primer, rattle can enamel gloss white, then polyurethane clear coat.

The white I used was this: https://www.bunnings.com.au/fiddly-b...paint_p1580053
The polyurethane I used isn't available anymore, but it was a British Paints Polyurethane Gloss spray.

I remember I did at least 3 coats of clear. However after a couple of years intermittent playing, the clear coat has rubbed off quite a lot of the body. Its given me doubts about the durability of polyurethane, but is this just a bad brand of clear spray? Or cheap/incompatible undercoat?

[McCreed]

Its given me doubts about the durability of polyurethane, but is this just a bad brand of clear spray? Or cheap/incompatible undercoat?

Incompatibility issue.
An enamel base coat should have had an enamel clear coat (or no clear at all). I've never done it, but AFAIK, enamel can simply be levelled and polished on it's own.

Polyurethane is best used over stained or dyed finishes. Whether oil, alcohol or water based. I have been using poly (satin) exclusively on my necks for a couple of years now and doubt I'll use anything else in future. I also have a couple of necks that are poly from as early as 2008 that are still wearing just fine.
As for bodies, I think I have two that are poly and they would be from around 2017-18, again, no issues.

[Rabbit]

Not all polys are created equal, and you realy need to check the can to see what substrates are suitable and what prep is required. With rattle can finishes, the coats are generally thinner so you often need more coats to get the same thickness as spray gun finishes.

[Simon Barden]

I’d also have used more coats. To get a smooth, glossy finish, sou need to sand the poly flat, then work up through the wet snd dry grades and then polish. This will reduce the poly thickness considerably, so you need to put a few more coats on to begin with in order to leave a decent protective thickness of it remaining once polished. Six applications should do it, as long as each application consists of three passes (e.g. horizontal, vertical then horizontal again) with the spray just close enough to ensure it goes on wet, not dry, but not so close that it pools and runs.

If you are spraying poly or acrylic clear over a matching colour coat and you’ve let the colour coat dry well before spraying clear, then it’s a good idea to sand the finish to allow the clear to key in to the colour. If the colour coat has too hard and shiny a finish, then the clear can have trouble adhering to it. It’s also a good time to sand the colour finish flat so that you have a known level base for the clear to go on. You then know that once the clear coats are on, you have minimal flat sanding to do before you can polish it.

Nitro is a bit different as the paint/lacquer solvent is so strong that it will melt the top surface of the nitro finish below so you will get adhesion between coats.

As McCreed says, best to stick with one type of finish type throughout.

And it always bears repeating that (unless you are using a catalysed spray finish) you need to let the finish dry and cure well to get the best results when polishing. Two weeks is a good minimum period, though if you can wait longer, then do so. A still-soft finish won’t take a good shine.

[Rabbitz]

Or the other solution is to build more guitars so the wear is spread out amongst the herd...

[kman42]

Or the other solution is to build more guitars so the wear is spread out amongst the herd...

an innovative and highly appealling solution!