Originally Posted by AXracer

If the bed floor is solid planks, then fourtyniner is correct use the same finish (I recommend 100% pure Tung oil, I have posted several times about this, do an advanced search on my user name.) equally on all 6 sides. Clear film finishes, i.e. typical hardware store products, are all destroyed in short order by UV which will leave it a mess. You never want to use a water tight sealant like undercoating or bed liner on wood, especially on the bottom. The wood has to be allowed to breathe to prevent mold and milldew (dry rot).

Originally Posted by Tyler S

OK, thanks everybody! AX if I understand you correctly (I read many of you posts on this topic) you recommend staying away from varnishes all together, just go with tung oil? And also, I gather you're not a fan of oak because of its poor performance with UV and its moisture wicking properties, is that right? I have oak (it came with my new bed) but I am thinking of switching to cherry.

Originally Posted by AXracer

Correct IF you are using 100% pure tung oil, NOT "tung oil finish" such as sold by Watco and others in the big box DIY and paint stores/departments. "Tung oil finish" is nothing more than polyurethane varnish thinned with solvent and containing a minimal amount of tung oil, just enough to be able to put it on the label. A marketing ploy used to sell a smaller volume of polyurethane varnish cut with much cheaper mineral spirits (as much as 50%!) for a premium price than charged for the same amount of uncut varnish! Pure tung oil is sold by specialty stores catering to the professional/serious hobbyist fine woodworker. It is expensive since it is pressed from the nuts of the Tung tree rather than being mixed up from chemicals in a vat in a factory, (just like fine extra virgin olive oil is more expensive than cooking spray), but it is a much better product to finish hardwoods with. Pure tung oil sells for about 20.00 a quart at stores like Woodcraft, Woodworker's Supply, and Klingspore, but it does go a long way. Let me correct and clarify my issues with oak: Oak is a very open grained porous wood, so it takes a lot of finish to seal the large exposed cells. Most people get tired of applying finish long before the grain is filled, leaving it able to absorb water which expands the cells. The expansion will cause any film finish to develop fine cracks or crazing, and lifting the finish, letting in even more moisture. Oak's high tannin content when in contact with moisture forms a very acidic solution. (Traditionally a solution made from oak bark boiled in water to extract the tannin was used to "tan" leather, Tanning refers to the darkening and change to the leather in contact with the tannic acid solution) Tannin when exposed to air and UV causes the poly finish to deteriorate and the oak wood to turn black. Your choice of (American black, not sour or Brazilian etc) cherry wood is a good one. Cherry heartwood is a dense fine grain wood that takes a pure tung oil finishing beautifully and when exposed to UV slowly darkens to the beautifully rich reddish brown color we woodworkers know and love. (PLEASE don't stain it, be patient and let it color naturally.) However if you apply a film finish like poly varnish the darkening will stop dead in it's tracks and it will never develop it's beauty. Cherry sapwood finishes to a pale yellow color which can give a beautiful contrast to the heartwood.

Originally Posted by 49f3dls

This may help you decide- I followed Ax's instructions for the tung oil and this is how it looks after three coats. The man knows what he is talking about as far as I'm concerned. Black walnut with tung oil only!

Originally Posted by AXracer

Correct IF you are using 100% pure tung oil, NOT "tung oil finish" such as sold by Watco and others in the big box DIY and paint stores/departments. "Tung oil finish" is nothing more than polyurethane varnish thinned with solvent and containing a minimal amount of tung oil, just enough to be able to put it on the label. A marketing ploy used to sell a smaller volume of polyurethane varnish cut with much cheaper mineral spirits (as much as 50%!) for a premium price than charged for the same amount of uncut varnish! Pure tung oil is sold by specialty stores catering to the professional/serious hobbyist fine woodworker. It is expensive since it is pressed from the nuts of the Tung tree rather than being mixed up from chemicals in a vat in a factory, (just like fine extra virgin olive oil is more expensive than cooking spray), but it is a much better product to finish hardwoods with. Pure tung oil sells for about 20.00 a quart at stores like Woodcraft, Woodworker's Supply, and Klingspore, but it does go a long way.

Let me correct and clarify my issues with oak: Oak is a very open grained porous wood, so it takes a lot of finish to seal the large exposed cells. Most people get tired of applying finish long before the grain is filled, leaving it able to absorb water which expands the cells. The expansion will cause any film finish to develop fine cracks or crazing, and lifting the finish, letting in even more moisture. Oak's high tannin content when in contact with moisture forms a very acidic solution. (Traditionally a solution made from oak bark boiled in water to extract the tannin was used to "tan" leather, Tanning refers to the darkening and change to the leather in contact with the tannic acid solution) Tannin when exposed to air and UV causes the poly finish to deteriorate and the oak wood to turn black.
Your choice of (American black, not sour or Brazilian etc) cherry wood is a good one. Cherry heartwood is a dense fine grain wood that takes a pure tung oil finishing beautifully and when exposed to UV slowly darkens to the beautifully rich reddish brown color we woodworkers know and love. (PLEASE don't stain it, be patient and let it color naturally.) However if you apply a film finish like poly varnish the darkening will stop dead in it's tracks and it will never develop it's beauty. Cherry sapwood finishes to a pale yellow color which can give a beautiful contrast to the heartwood.

Originally Posted by ALBUQ F-1

If you are talking about treating the underside, per your OP, how much UV is it really going to see? What it will see is 90wt gear oil slung off the pinion seal, grease flung off the U-joints, and likely a fair amount of heat from the exhaust where they jump over the axle. I wouldn't spend too much time or money on it.

Originally Posted by Tyler S

Hey Chuck, I am looking into getting the cherry here in the next few days at my local Woodcraft store. Incidentally, the price they quoted me didn't seem too bad: $180. Anyway, I was discussing the tung oil approach with the guy at the store and his only reservation about using %100 tung oil was that it has no UV inhibitors. Above, you explain the UV exposure helps darken the cherry to a beautiful tone, but does it also damage the wood over time? Obviously UV is a huge issue out here in AZ, while rain is less so. It does rain, especially in the winter and during summer thunderstorm season, but not like what you get in your neck of the woods. He was suggesting I consider a product called Outdoor Oil (General Finishes Outdoor Oil is an exterior-rated oil. | General Finishes) which I'm sure you're aware is a linseed oil with UV inhibitors. Evidently, like tung oil, it is not a varnish and you can reapply it in the future without stripping/sanding. What is your take on that product and the UV inhibitor issue? Thanks! Tyler