I use a belt sander to reduce the grey wood down to the color of mahogany. (Always sanding with the grain).
The grey weathering runs deep, but mahogany is such an elegant wood, I think it's worth the loss of fractional inches.
There must be a primer to bring the dry wood back to varnish; however, if you're thinning the first couple of urethane coats, I'd think that'd be enough.
The first 3 coats were thinned out some.
I thought about going further down on the grey, but honestly it was running real deep. Plus the wood had a lot of cracks including a crack that was about 8" long and 3/4 the way through the wood. This dagger board was more of a practice for me as it is 'extra'.
Anyway, I stopped sanding because it would take a while and I had other things to deal with that were more important. There is always a balance.
One the other hand, the rudder from my sons Sunfish was pretty decent has a real nice color so I worked more time on it to make it look pretty.
I thought about going further down on the grey, but honestly it was running real deep. Plus the wood had a lot of cracks including a crack that was about 8" long and 3/4 the way through the wood. This dagger board was more of a practice for me as it is 'extra'.
Anyway, I stopped sanding because it would take a while and I had other things to deal with that were more important. There is always a balance.
One the other hand, the rudder from my sons Sunfish was pretty decent has a real nice color so I worked more time on it to make it look pretty.
I started a thread on this subject recently. I’d have filled the crack with thickened epoxy, clamped, sanded, then coated the whole rudder with epoxy. SC and tag and others have also encouraged a horizontal screw for reinforcement, as well.
No need to thin the varnish, at least I’ve never needed to.
I read mixed things about thinning. Figured it would not hurt.
After working with the Helmsman spar urethane i do not see it as critical.
I got a Purdy syntox brush but I do not like how it lays down the varnish. I have a used Amber Fong brush that does a nicer job. But it had some white paint in it that I did not know was there and it started coming out the second time I used it. I will try a cheap Ace wall brush next.