Polyurethane finish questions

[JeffHGW]

Hey,

First post here so might as well introduce myself. I’m Jeff, been building guitars in Rotorua NZ for the last few years and lurking on this forum from time to time. I’ve been spraying mirocat 3220 on all my guitars, there’s some things I don’t like about it so I’m thinking of switching to mirothane 5555, but I’ve got some questions. Hopefully there’s some people here with experience with that product who can chime in.

- Looking at the data sheet, they have a range of mix ratios (1:1, 2:1, and satin has a 3:1 option), some of them with several options for hardeners. I’m a bit confused as to which combination is best for me. What are the considerations when choosing?
- What speed thinners are people using, in what climate? What ratio? Why?
- What does a typical finishing schedule look like? Number of coats, time between coats, when to sand etc.
- How long do the isocynates and other nastys take to gas off? When is it safe to approach without PPE?
- How do touch ups, repairs, and drop fills work?
- Spray gun cleaning compared to lacquer? Obviously don’t let it harden in the gun, but does the gun have to be cleaned especially well each time?
- Do you guys/gals use the 5545 sealer or sealer in general? Why or why not?
- One of the things I really don’t like about the 3220 is the sinkback which occurs in the months after spraying. How much sinkback can be expected with the mirothane?
- Another thing I don’t like about the 3220 satin is how quickly it glosses up. How well does the satin 5555 hold up in comparison?

Thanks

Jeff

[Allen]

I use Mirothane now going on for 5 years.

Their website and instructions are very confusing, and I believe its because their product can be applied in so many ways for different purpose and application. My local distributor has laid out what would be most appropriate for the vast majority of his clients.

The Hardener / Catalyst is the same for both products. #5747
Solvent is their Mirosol. You need to pick one for your conditions. I live in Cairns so I use #1218 Slow

Mirothane #5545 Marine Grade Sealer - Mix Ratio 2:1 + 25%
Mirothane #5555 Clear Marine Grade Topcoat - Mix Ratio 1:1 + 25%
Clean up is with a good lacquyer thinner / gun wash.

The product is fairly easy to apply if you are competent with a spray gun. I allow 10 minute flash off between coats. But that depends on your conditions.

Day one is their sealer. Usually 3 coats is good for this step. You can level sand and go onto clear coat in about 4 hours, but I always leave overnight.

Day 2 is Clear Coat. 4 - 5 coats. Depending on how good your pore filling and prep was done, you might be able to stop here.

If required Day 3 is a fine level sand and if you go with P600 grit you might be able to get away with 2 flowing coats and call it done.

I let it cure for 3 days and then level and buff. I have done it in one day but it's not really cured hard enough to take a good buff. It would also present a possibility for some sink back as it hasn't fully cured / shrunk back.

Sink back is caused by solvents gassing out of the 3320. It never stops gassing off, just slows down over time. With a catalysed product the only sink back you are going to get is from pore filler sinking back if it wasn't cured fully or of a paste type that hasn't fully dried yet. You will get solftwood soundboards showing grain lines as they move a fair bit with humidity if you've applied the finish nice and thin as it should be.

I've only done a couple of satin finishes with Mirothane. It seems to stay satin longer than the lacquer equivalent, but all of these products will eventually gloss up with use.

[Mike Thomas]

Thanks Jeff for asking the questions. And thanks Allen for you very helpful response.

[JeffHGW]

Thank you Allen, that clears things up for me.

[Allen]

I'll add a few things to this post.

• You need to spray a wet flowing coat. What you see off the gun is pretty much what you'll end up with when it's cured.

• As it's a catalysed product, only make up what you can use for the job.

• Depending on your temp and choice of solvent, it's quite possible that it will start to thicken and go off by the time you are on your 3rd or 4th coat. You just cant spray it when the viscosity changes that much and you'll end up applying a coat that is far too thick. I only make up to apply 2 coats, and then another mix for the other 2.

• If you have a small pin hole or gap on binding etc. show up after any of your spray sessions you can drop some thin CA in it and then sand some of the clears dust into it. Drop CA on it again. Repeat until filled. Your next coats of clear will turn it transparent and in 99% of cases totally invisible. If really small you can even get away with this after your last spray session and getting ready for your level and buff out

• Drop fill repairs with some of the new CA glues that are designed for this should work. Not tried it myself on anything that would be easy to see like middle of soundboard. I've tried using the catalysed product to drop fill but I always end up seeing a ghost outline of the repair. Perhaps I'm ultra critical though.

• To achieve invisible repairs to the finish they should be treated as a panel refinish unless it's very small and you can hide the edge you are going to end up with. Such as on a binding and you could soft mask either side and buff smooth later. I would do the same with lacquer as well, so really no difference here.

• If you get runs / sags in the finish scrape them flat with a razor blade prior to level sanding. You won't be able to level them sufficiently with a block without cutting through next to the run / sag. Depending on the timber this might be visible when recoated.

[JeffHGW]

Excellent, thank you once again!