Applying sealant on top of sealant
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1. I was air sealing a cavity outside using dynaflex ultra. Ran out in the middle of the job. A few areas need a thicker bead. Can I apply new sealant the next day on top of sealant from the prior day? I know it’s generally advisable to scrape out old sealant. Just not sure if that means 1 day old sealant.
2. Some areas need foam, but I applied a little sealant first to seal small gaps. Can great stuff foam be applied on top of dynaflex ultra?
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A lot of the sealants, including most acrylics, can get another layer, especially if it was recently applied. The exception is silicone, the bead and any residue will need to be completely removed before any sealant, including the same silicone product will adhere correctly. There are a few silicones that will bond to itself, you'd want to follow the manufacturer's instructions in that case.
Foam should be able to come in contact with a sealant, but I would only butt it up to an edge, for instance if a narrow gap was sealed using a sealant, a wider gap required foam. Some manufacturers have information on their website if there is a compatibility issue between products.
I would scrape the old stuff off first. Even if the new sealant is able to bond to the old sealant, you'll still end up with a physical deformation there, which may impede sealing when everything is squished together, if this is a sandwich-type assembly (such as between plywood and framing). If it's an exposed bead of sealant in something like a gap accessible from the surface, then it's not nearly as critical, because a little bulge in the sealant won't affect anything.
I've never had a problem with canned foam adhering to sealants, but I would let the sealant dry first before applying the foam.
Bill