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Because I don't know what I am doing, I put some oil based SPAR urethane on some book shelves. It's finally beginning to harden up and I want to put a few more coats on, but this time with oil based REGULAR urethane.

Am I likely to run into problems if I just give the spar coat a light sanding and continue on with the regular urethane or should it strip off the spar coat?

Thanks,

Lars
 

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Log dog
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I'd strip it off. Spar will always be soft. Adding regular polyurithane will only make it worse.
Spar is mainly for outdoor use. Spar is marine poly. It stays soft to move with temp and changes of whether.
 

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If your spar is dry I think I would just sand it with 220 grit paper and put another coat of the same spar over it. I very rarely need more than two coats of the spar varnish.

Spar varnish is formulated to be more elastic due to the fact it is made to be used outside in the weather. If you put a interior urethane over a spar the urethane will more than likely crack because it will not breath at the same rate as the spar. If you continue to use the spar you will have a finish that has the same hardness as a front door on a house. I don't think it merits stripping the finish and starting over.
 

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Old School
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I'd strip it off. Spar will always be soft. Adding regular polyurithane will only make it worse.
Spar is mainly for outdoor use. Spar is marine poly. It stays soft to move with temp and changes of whether.
+1. :yes: That's what I would do. Spars have a greater oil ratio than an interior varnish or oil base polyurethane. Coating over it with either of those would maintain that softness/flexibility as a base coat. Adding the same thing would be the same as what you have now.





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Stripping that spar off is a lot of work. Its a messy job. If you are using Minwax Helmsman the finish will hold up fine in a bookcase. I used to use if for just about anything. I just quit using the stuff because it changes color quite a bit over time. My bigger question is when you say "It's finally beginning to harden up." Most spar urethanes will dry in 3-4 hrs. If is taking much longer than that there is something likely wrong with the finish. High End spar varnishes ( I wouldn't use these indoors) tend to take much longer to dry and stay much more flexible than spar urethane. I have used 100's of gallons of Spar Urethane on interior projects and have never anyone complain on how they have preformed for them on interior items.

Geoff
 
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