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exterior caulking

Hi Again

A while back I asked about the best caulk to use on exterior siding in Phoenix. The answers were great, if sometimes hard to find the products.

I am now in a position to begin work. I did a few test runs to see how the old caulk comes out and found a few things. The siding is a fiber board type (dark brown) and flakes or shreds when sc****d. It seems as if the original caulk was set not only in the seams but also between the overlaps of the boards.

I can get most of the material out from the seam, but obviously can't touch the overlap caulk. Will it be okay to caulk in the seams without doing so?

The second issue is that the original caulk was also laid in the indent part (1/2 inch +/-) where the boards join. This will be difficult to remove. Is it necessary to do so? Will the new caulk adhere if it isn't removed?

A third issue is the board material. The fiber shreds easily when the caulking is being removed. While I don't think it is a problem as it will be covered with the new caulk, I'm slightly concerns that because it shreds, the new caulk may not adhere well.

Sorry for the long post. Appreciate any help you offer.

John Pisacano
Phoenix, AZ

Re: exterior caulking


I assume you are talking about verticle siding. Verticle siding does not actually need that much caulking if it is installed correctly. The overlapping and batten boards are sufficient to keep rain out. The underlying rain screen (hopefully there) will keep out any moisture that should possibly find its way in and will stop air infiltration. In my opinion, the average house is over caulked by overzealous "mad caulkers" :)

When you remove caulk and have bare wood exposed, ideally that wood should be spot primed first. In your case, with the fibrous siding, I would use a quick dry oil primer to set and solidify the pulp before caulking.

My first choice of caulking is a urethane based. There are many good brands out there: Sonneborne UP1, Vulkem, Quad, etc. These are national brands which should be available locally. Your local big box store will for sure have one of them.

Urethanes remain elastic, have incredible adhesion and are immediately rain proof after application ( important here in the rainy Pacific-North-West, but probably not so in Arizona). Curing might be a little slower in dry climates because urethanes actually need a little moisture for curing.

Re: exterior caulking

Thanks. Never thought of priming it. Would a shellac based primer do? or should it be oil based?


Re: exterior caulking


Shellac is not the best sealer for exterior use. Ironically, it seals to well and does not breath. You would be better off with a quick dry oil primer like CoverStain.

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