I have tried a number of commercial seam sealing products, all of which tend to be expensive for the small volume you get.
Commercial seam sealers divide into two basic groups: water based emulsions and solvent type. I have not had much success with the water based emulsions but the solvent type have been OK. However, looking at their composition (easy to find, just search for MSDS (material safety data sheet) and the product name) all seem to consist essentially of plastic dissolved in a strong solvent like acetone, toluene or methyl ethyl ketone (MEK).
It struck me that this is the composition of plastic glue, so recently I have been using glue sold to plumbers for PVC pipe like this:
This one is clear (some are coloured), flows easily and costs about $5 for 250 ml compared to $18 for 45 ml of typical commercial seam sealer i.e. less than a 20th of the price.
Unlike water-based sealers which seem to take hours to dry completely, this dries in a minute or so.
I have used it on a variety of fabrics including PU coated and it sticks and seals fine. That said, there is no guarantee it will work safely on every fabric. I have my doubts about silicone coatings.
This brand comes with a brush but it is too big for seam sealing so I usually apply it with a cotton bud. As with other seam sealers, a couple of coats are a good idea.
- These contain powerful solvents and so have potential to do a lot of damage. Test them on a hidden/non-critical area first. Don't blame me if it dissolves your gear, makes it shrivel up or whatever!
- This is glue, what glue sniffers sniff. It may make you feel light headed, nauseous etc so apply it outdoors. Used sensibly there should be no problems. (Avoiding this is the reason for those water based seam sealers.)