Question of compatiblity of finishes

[wood2woodknot]

I looked this up before but can't find the answer this time. Are urethane and polyurethane finishes compatible - can I use one over the other?

I remember seeing a label saying "Do not use over polyurethane," but can't remember the product.

I am using a Minwax Polyshades wipe-on finish for the base coat and don't know f I can just use a clear urethane finish I already have (Arm-R-Seal) for a topcoat; or do I have to get a poly topcoat? Prefer not to buy anything more if I don't have to.

[JR1]

There is only one pretty widely universal incompatibility that I know of and that is waxy Shellac and NC lacquer. The only way to be sure is to try on a test piece. Be aware that Arm-R-Seal is slow curing so you will not be 100% certain sure for a couple weeks. I have not hit incompatibilities between urethanes, acrylics, vinyls and polyesters. But I have only tried a very small subset of those available. The only way to be certain sure with a new finish combination is to test it.

[jteneyck]

If you are specifically asking about Arm-R-Seal going over MiniWax Polyshades, I think it will be fine as long as you scuff sand the Polyshades with 325 or 400 grit first. I have no idea why Arm-R-Seal says Oil and Urethane on the can. It's an oil based polyurethane varnish, just like Polyshades. I'm not saying it's the same urethane resin in both, but they both are varnishes made with some type of oil and urethane resin.

I think you will be fine.

John

[Howard Acheson]

>>>> Are urethane and polyurethane finishes

compatible

Sure, they are identical finishes. Just two different words for the same finish.

But a couple of things. It's never a good idea to intermix different manufacture's finish products. Also,urethanes have rather low adhesion. If you are applying one urethane over another, be sure to carefully scuff sand.

[Howard Acheson]

Poly, urethane and poly urethane are all the same thing. Just different names for the same product.

However, it's best not to intermix finishes from different manufacturers. Also urethanes have low adhesion so if you are over-coating a prior urethane finish be sure to thoroughly scuff sand the lower finish.

[Howard Acheson]

<<<< There is only one pretty widely universal incompatibility that I know of and that is waxy Shellac and NC lacquer.

You are mistaken. The incompatibility is applying an oil based an oil based poly over a shellac containing it's natural wax. Both shellacs are completely compatible with NC lacquer.

[JR1]

I tried waxy shellac (BYSAKHI BUTTON [a very waxy shellac]) and then applied various topcoats and NC lacquer showed no adhesion at all. After a weeks time it could be wiped off with a paper towel. None of the waterborne that I tried had any problems

As for oil based poly the only one that I use is Arm-R-Seal and I have never tested it over waxy shellac.

[wood2woodknot]

Thanks, guys. I already did a small sample with the Arm-R-Seal and it seems okay. Will do another test before I'm done with the PolyShades base coat. But at this point it's "on with the show."

[Richard D.]

I'd like to point out that up until this year JR1 never pretended to have used Arm-R-Seal like he pretends that he uses many finishes. (Those that follow the forum know he doesn't do any woodworking other than in his imaginary world.) After John (Jteneyck) posted several tests of Arm-R-Seal and other finishes, where the Arm-R-Seal did very well, JR1 started touting it's superior qualities as if he has hands on experience (which we know he does not).
I'm sorry if anyone feels I am picking on JR1 but I think it's important for new members of the forum to be aware of his behavior so they are not fooled into thinking he is an expert based on the amount of replies he makes. Members of the forum have been requesting photographs of any project he has completed in the last several years and to date we have only seen a blurry photo of a piece of walnut with some shellac on it. Considering that JR1 also claims to be a professional photographer the situation is curious to say the least.

[Trusty Rusty]

Say it ain't so!
Rusty