If you have a real lumberyard around buy some 2x lumber that is long enough in a single piece and have it planned down to thickness.
If you cannot find a single length piece, treat the cut ends with Minwax Wood Hardener to prevent water absorption into the end grain. Unlike paint it soaks in and then hardens. Apply as many coats as it takes to get the slightest start of surface buildup, then prime and paint.

Brickeye,

I think you have something on the sealer at joints, this is where the failure takes place (yes one 20+foot length would reduce that problem to the just two ends). I have use Minwax finishes inside for years, and like it very much (albeit the formulation has changed over the years to produce less air polution). In any case, sealer along the miter joint may be a good idea too. The use of the TUF boards too would solve the problem as they are not wood.

Brickeyee,

Coming at you again. While at WalMart tonight I looked in the Minwax section and didn't see any wood hardener. Is this something I should be able to find at Home Depot or similar store?

OK, if I'm "talking to myself" too much I'll stop soon.

I did find Minwax Wood Hardner at Home Depot, at about $8 per for 8 oz (from memory, I didn't purchase yet). Reading the package it seems it is much more than I expected as it advertises itself as a "fix" for dry-rot. Again, not sure it said "dry rot", speaking from memory again. Is says apply several coats in quick succession, or until the affected area becomes shiny, then after drying/curing fill area with Minwax wood filler. The wood filler is also rather expensive, but if it works it could be well worth the cost. The reason I'm considering replacing trim boards is that there was some dry-rot (water damage) along about a 8" section of the miter joint to the trim board on the other side (of the hourse, these are on the house corners). The damaged area is about 12' off of the ground, so a patch isn't very noticable. I'm now thinking about buying the Minwax problems and giving them a try rather than replace all or part of the 20+ feet of corner board.

My experience with Minwax Wood Hardner is that it lasts only a couple of years, and cracks begin to develop within a few months of application. I have had better results with Abatron Liquid Wood. It is a two-part epoxy product that appears to form a permanent repair.

They also make an epoxy wood filler product that is white, and you mix the two parts and apply like putty. I don't like that product, however. It is extremely light weight, which tell me that it is mostly inert filler, plus it is snow white, which would be fine if I were going to paint the repaired wood white, but I prefer something closer to natural wood colour.

Something that I have found to work extremely well is to mix up a batch of Liquid Wood and then stir in fine sawdust until a thick paste is formed. It can be applied with a putty knife, and when hardened it is only slightly darker than the wood that is being repaired. I have one exterior window sill that was repaired and has gone through an entire season, with hot, cold, dry and wet conditions, and so far, no sign of cracking or a gap between the filler and the wood.

I would advise priming the rotted wood with pure liquid wood and allowing it to penetrate before adding the filler.

Liquid Wood is not cheap; it runs about $165 for two gallons (1 gal each hardener and resin). I can go to the local lumber yard where they manufacture doors and frames, and they will give me all the sawdust I can carry away, since that makes less that they have to clean up and dispose of.