I had a similar problem -- treated timber stair/railings on a new house. Filled nail holes with latex caulk, and painted with latex exterior semi-gloss enamel. Some shrinkage of the caulk, so gave it another shot, and the second coat of paint. So far, so good, and one year into this. Only problem is where I hit things with my string trimmer when mowing the yard.
I've always had good luck with Minwax high performance wood filler. It doesn't shrink. You can't put too much excess on though because the stuff dries as hard as a rock and can be tough to sand. You get it at the borg.
Lots of wood workers use Bondo. Sands easy (if you overdo it), cures really quickly, doesn't shrink and can fill major voids better than anything else on the market. Compatible with just about any kind of paint. Best of all, low price. Just remember to use small batches and work fast, best adhesion is early in the cure cycle. HTH
Why anyone would use Bondo for an outdoor application is beyond me, as it is VERY hygroscopic. Just ask anyone that's lived in the rust belt; if you patch a rust hole in a car body with Bondo and don't thoroughly seal/paint both the visible side and the backside of the repair almost immediately, the rust will come back with a vengeance, worse than ever.
Nate, you're a little out of touch with current autobody repair and fabrication. For example, if you had ever seen an episode of the TV shows about Boyd Coddington's or Skip Foose's hot rod show cars you would have noted Bondo being applied to the entire car surface and then sanded to the final contours before being primed and painted. Bondo is widely used because it works, and what you think you are seeing is simply the product being slapped onto carelessly prepared or unprepared surfaces. It's used in new boat fabrication as well, so your statement concerning hygroscopicity can't possibly be correct. On a more direct experience level, I have some porch posts repaired with the Minwax restoration product (which is actually a polyester filler just like Bondo) that have been exposed to the elements for about five years with no signs of failure. Try Bondo some time on a tough project and I think you will be convinced. Cheers,
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