Harder Curing Polyurethane

[017_017]

Hi,

I've been doing some test pieces with Minwax Wipe On Polyurethane (oil base), and I'm finding it isn't curing anywhere near as hard as I expected it to. While it certainly seems tough, it's easily dented and seems quite soft. I know it wasn't going to get as hard as a professional 2K system, but I thought it would cure a little harder than this.

Would going for an interior system intended for floors give me a harder finish, or would I just be chasing myself in circles?

Thanks.

 

[Cat MacKinnon]

well, it depends on how long you let it cure for. even though most finishes are "usable" within a few days or a week, it can take several weeks for them to fully cure and reach full hardness. there are some finishes that continue curing over the course of years even! also, oil-based finishes usually take a lot longer to cure than water-based. poly is also supposed to be slightly softer than something like nitro, so that it doesn't crack and check as the wood expands and contracts (although it's still hard from a touch perspective.)

you also need to make sure that you're following the instructions on the can. unlike lacquer, polyurethane doesn't "burn" into the previous coats, so you need to sand or scuff between every coat. not only is this important so that they all stick together properly, but the scuffing helps "open up" the previous coat a little so that more solvents can evaporate. finally, you have to make sure you're putting on really thin coats, because trapping thicker coats under subsequent ones can make the finish take forever to cure/dry properly, if at all.

if you've followed the instructions properly, the best thing you can do is give the finish several weeks to dry, especially because you used oil-based poly. but even people who spray lacquer generally wait a month to six weeks before they start polishing out the finish and assembling the guitar. unless you get into 2K or UV-cured finishes, there's really no such thing as a "quick" finish that fully hardens within a few days.

i suppose you could use a hardwood floor finish, but i think the reason people avoid those is because they go on really thick and tend to not look very good (very plasticky looking.) i don't think it's really worth going down that road.

 

[jstream]

You might take a look at EmTech's EM9000 which I believe is classed as a poly

 

[joeford]

i've used the minwax poly a couple times. that stuff will be hard as a rock after 2-3 weeks. you just have to be patient

 

[017_017]

It's been at least 4-5 weeks. Strangely, I filled a cup about 5mm and left it outside to cure, and it's harder and almost impossible to mark than what's on my pieces of timber (maple and Tasmanian oak). The timber I applied it to is/was dry. The finish on the maple is slightly harder (and has been curing for about two months now), but is still quite easily marked. Unless I'm marking the timber itself, and the coating is just "going along for the ride"? Or I’m cursed?!

I'm not in the USA, so I am going to have to go local if I decide to go with another route. StewMac offer a water based Target Coatings 7000 HBL product that can be shipped here, but the shipping costs are close to 2x the cost of the product. Locally, the most common systems are Wattyl Estapol and Cabot's CFP/Cabothane. Mirotone Mirothane/Mirotec should be simple enough to get, too. In regards to application, I prefer to wipe or brush; not spray.

Thanks again for all the comments.

 

[joeford]

the brush on stuff is equivalent to like 10+ coats of spray. it takes a lot longer for that thickness to dry. again, i've used it with good luck... but it takes patience. also, the top layer will always be a little soft until you sand it down a bit, like cat mentioned above. for the best results and a mirror-like gloss finish... you have to let that stuff dry, sand it down with some fine grit sandpaper, then bring it back to a shine by buffing. i know it looks glossy and pretty as soon as its dry, but you really need to sand it out and buff it to get a hard finish

 

[017_017]

I'll persist with the Minwax. Thanks again for the help.

 

[Tom Pettingill]

... The finish on the maple is slightly harder (and has been curing for about two months now), but is still quite easily marked. Unless I'm marking the timber itself, and the coating is just "going along for the ride"?

Wipe on poly goes on too thin to ever expect any kind of armor shell from it.
Think of it this way, wrap your guitar in aluminum foil and then whack it with a hammer.

 

[dbickford]

I used the Minwax wipe on poly on a build. I ended up doing 14 coats oven the span of 5 days. Sanding very lightly with 320 grit between coats. I think it came out nicely. As for durability, i think it takes abuse as well as a nice lacquer paint job.

 

[dbickford]

 

[017_017]

Thanks again for all the comments.

 

[Elias Graves]

Defthane. As much as I dislike their lacquer, the poly is hard as steel. They use it on the space station. It's good stuff.

 

[crazy_aussie]

It's been at least 4-5 weeks. Strangely, I filled a cup about 5mm and left it outside to cure, and it's harder and almost impossible to mark than what's on my pieces of timber (maple and Tasmanian oak). The timber I applied it to is/was dry. The finish on the maple is slightly harder (and has been curing for about two months now), but is still quite easily marked. Unless I'm marking the timber itself, and the coating is just "going along for the ride"? Or I’m cursed?!

I'm not in the USA, so I am going to have to go local if I decide to go with another route. StewMac offer a water based Target Coatings 7000 HBL product that can be shipped here, but the shipping costs are close to 2x the cost of the product. Locally, the most common systems are Wattyl Estapol and Cabot's CFP/Cabothane. Mirotone Mirothane/Mirotec should be simple enough to get, too. In regards to application, I prefer to wipe or brush; not spray.

Thanks again for all the comments.

I've just finished an ES335 clone with Cabothane spray gloss over black enamel. When I say finished, the spraying's done and its now curing. Its been 3 weeks and I'm going to leave it for another week before polishing. I found that multiple light coats of Cabothane works best and wet sanding with 600 grit paper between coats helps the next coat stick properly and also smooths the surface as you go. I tried a heavy first coat and got lots of orange peels which then had to be sanded out. Not ideal.......FWIW