I did a 1950 Merc a few years back, the truck always sat in the morning sun. Anyway Marine varnish was used, and as far as I was concerned this was a waste of both money and time. I thought this stuff would be one of the best finishes on the market. I followed the instructions to the T, and it came of just like the clear coat of the early 90s paint jobs, at least my 94.

I was talking with a friend one day who was into wood refinishing, and mentioned my problem to him. his advice was to remove the marine coating and prepare the wood for a good coat, of the following. Boiled Linseed Oil, and diluted with Varsol, or paint thinners, "NOT LAQUER THINNERS", apply three to four coats. This will get right into the wood and give it a new lease on life. Then as another person mentioned use a stain of your color choice, I myself prefer the Watco Danish Oil, in a Dark Walnut. The Minwax is also a great product, I just happened to like the Watco brand, and it was on hand. After a few liberal coats of this is applied, and dried then comes the weather seal or UV Urethane. A few coats of this with a light sanding betweet to remove the air bubbles, and you have got one helluva nice bed. there is a bit of work involved, but it is well worth it in the end. Hauling sand or gravel after this not recomended.

 

Take a look at this link on bed wood finish suggestions.
http://www.mar-k.com/wood_finishing.html