Sealing wood cracks? ? ?

I'm replacing some exterior flooring on a porch and want to seal the joints and cracks with a silicone caulk.
Problem: Some of the joints are quite large. I need to put some sort of filler in the large spaces before I use the silicone.
I suppose I could stuff some styrofoam in the cracks.
Does this make sense?
Any better ideas?
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Ray Jenkins wrote:

They make backer material for that purpose. You also might want to consider polysulfide (Thiokol) rather than silicone. It is used on boats and is a lot easier to sand/cut flush than silicone. Paintable too.
-- dadiOH _____________________________
dadiOH's dandies v3.0... ...a help file of info about MP3s, recording from LP/cassette and tips & tricks on this and that. Get it at http://www.gbronline.com/xico / _________________________________
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Thanks dadi --
Will look into polysulfide. Guess it's available at paint stores.
Would the styrofoam be okay for filling large cracks before caulking? I think styrofoam is nondebiogradable.
I think I made a mistake in earlier caulking by using Durham's Water Putty. I gather it holds moisture, hence causes rot.
-- Ray

consider
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Ray Jenkins wrote:

Rather depends on the paint store. I rather doubt it though. http://www.google.com/search?hl=en&ie=ISO-8859-1&q=thiokol+polysulfide ____________________

Beats me. Probably but I don't know. ________________________

Yes. Doubt if you could have found anything worse.
-- dadiOH _____________________________

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