Why mix spar urethane and mineral spirits

[zlamb0002]

I have been researching how to waterproof my teardrop and I have seen many people say spar urethane and mineral spirits. Why both, is the spar urethane good enough on its own?

[Nitetimes]

Thinning it down lets it penetrate better.

[GPW]

Zlab, wood is nothing but little tubes glued together...the mineral spirits thins the Urethane enough to get inside and seal the little tubes...once the tubes are sealed ,water can't get in... This mixture works very well on common cardboard too....probably even canvas or other fabrics too....encapsulating the fibers in waterproof plastic... We've used it for years on many projects ...stands the test of time
In "the old days", thinned oils were used to seal the wood/fabric... Anybody remember Linseed oil???

The secret to good waterproofing is getting it EVERYWHERE!!!!!!!!

[jay]

this is probably for the first coat only. the raised grain of the wood should be removed with fine sandpaper before subsequent coats.

some varnishes must be thinned with a proprietary solvent and some of the solvents are quite volitile. marine industry products especially

[48Rob]

It depends on the species of wood you are trying to seal.

Oak for instance, is Very porous and may take multiple applications of the thinned product.

A tighter grained species such as Maple may require only one or two.

GPW's explanation is a good one.
Urethane, varnish, paint, any oil based product that uses mineral spirit compatable solvent can be thinned this way.
Other products can be thinned with their blend of solvent.

I built a folding canvas bathtub a couple years ago.
I sealed the canvas with clear silicone caulk.
I used oderless mineral spirits to thin it down enough I could apply it with a brush.
Two coats sealed the canvas, and kept the water inside.
I could have smeared the full strength silicone over the canvas, but it would have been very thick and prone to cracking because it didn't penetrate into the fibers, but simply stuck to the outside.

The same concept drives the thoughts behind thinning varnish to seal wood.
If you can get it deep enough into the wood, the wood is actually sealed, which keeps moisture out.

Applying only surface layers of varnish is similar to wrapping the wood with clear plasic cling wrap.
It looks good, and will keep the water out...until it becomes nicked or damaged.
Once there is a "hole" in that protective layer, moisture attacks the exposed wood AND spreads underneath the layer of protection that is only sitting against the surface of the wood.

Rob

[Frosteez]

How much do I thin Spar Varathane- or will it say on the can? We aren't quite there yet but soon will be.

[zlamb0002]

I saw somewhere 3 parts mineral spirits, 1 part urethane, but I was wondering the same thing.

[tinksdad]

3 to 1 is what I used. Just be sure to coat every where. It is a thin mix, so it will run if you put it on too thick. Let it completely dry between coats. I put 3 coats on the bulk of the trailer, more on the edges.

[48Rob]

How much do I thin Spar Varathane-or will it say on the can?

For your final coats, full strength is best. Some products can be thinned, it will say on the can.

What has been discussed here, thinning the material, is NOT recommended by the manufacturers.

That is not to say you couldn't varnish a project with the material severely thinned, but it would be very difficult, and probably wouldn't look very good.

What is being discussed is a home brew to seal the wood before you apply varnish as recommended on the can.

I generally use 25% mineral spirits to 75% varnish, but it really depends on temperature, humidity, and the wood it is being applied on.
You may or may not need to thin it further.
50/50 is pretty darn thin!

The general idea is to get it thinned to near the consistency of water, so it can easily penetrate, and penetrate deeply.

Rob