Spar urethane finish question

[Mike Pittenger]

I’m building my daughter a cherry bedside table. I put a coat of BLO (Tried and True) on, let it dry for 5 days, then applied Helmsman Spar Urethane. The finish is very rough. Should the urethane be thinned before applying?

[Steve Schoene]

All varnishes need thinning these days. Start with about 10% thinner and it should level better. Of course, with any varnish you will catch a fair amount of dust nibs. These should be sanded (320 grit) between coats, and buffed off of the final coat. (600 grit followed by some rubbing compound for satin, or up to 1200 grit followed by polishing compound (or rottenstone) for a glossy finish.

I wouldn't recommend spar urethane for furniture, however. The defining feature of spar varnish is flexibility, which is needed on yacht woodwork. Consequently, it is rather soft and easily marred when used on furniture.

Remember that yacht owners always insist on soft soled boat shoes for guests. They may tell you it is to keep you from slipping and possibly drowning, but the real reason is that they don't want the brightwork dinged up.

[Prashun Patel]

Also, how many coats did you apply? The first couple of coats will leave your work rough until the pores are completely filled. Then it'll start building to glass.

[Mike Pittenger]

2 coats so far - but I've sanded both down pretty far to try to smooth it out

do I thin with alcohol?

[Prashun Patel]

Don't thin with alcohol. Try thinning with mineral spirits. Wait for Steve to verify, but I believe the prob is that you haven't applied enough coats. Adding mineral spirits or naphtha to the poly (at say 50/50 mix) will make it easier to apply and more forgiving when you level it out. But the roughness (I bet) is just bks you don't have enough on there.

Don't try to lay it on thick, though. Many thin coats better than 1 thick.

[Steve Schoene]

Shawn is right--mineral spirits or naptha for oil based finishes. (Alcohol is only used to thin shellac.)

At this point, I would shift over to a more suitable varnish for furniture. It should work OK over the Minwax stuff, particularly since you have sanded it pretty heavily. The best results would come from using a traditional resin varnish such as Pratt & Lambert 38, Cabot Varnish (not polyurethane, McCloskey Heirloom (if it is still on shelves). You can use an interior polyurethane, but it will be harder to achieve top quality results.

To brush on, start by adding about 10% thinner. You shouldn't have any problems with brush strokes or bubbles that don't level out. A good brush helps with varnish. Purdy makes a ox-hair mix that works well and is reasonably priced. You will be more likely to find it at a good paint store than at a big box store, which carries a much more limited part of the Purdy line.

Shawn is also right that it takes a couple of coats before the surface really can be leveled with sanding. So you should have smoother sailing going forward. However, you will always have dust nibs in varnish. Finding a place far from the workshop to minimize dust helps, but never keeps enough dust away for a really top finish without rubbing out.

You can also thin the varnish more heavily--up to about 50% thinner and apply the varnish with a cloth. No brush strokes, and the thinner coats set out of the dust a little faster. But it takes about three wiped on coats to equal 1 brushed on coat. If the wiped on varnish appeals to you we can go into some techniques for using it.