I've used diluted silicone (make sure you get the right kind) to seam seal my DIY hammock tarp, including a row of stitch holes that I made by mistake. works really well. mix it thin, have the section you want to do hung as flat as possible, for a whole tent you might have to do panel by panel. And then just paint it on. my theory is that by mixing it thin, you are adding the minimum weight to the tarp, and letting the solvent pull the silicone into the gaps. and more thin coats can always be added, a thick coat that ends up uneven won't make you happy either.
It may depend on the combo of solvent and silicone that you use, I found that the one I used gelled up in the mixing cup after about 30 min, it seemed to be chemically setting, as adding more solvent didn't help. But then as it dried out, it shrank down to the volume of silicone I'd started with. So only mix what you can use, and use disposable stuff.
Its at least worth the few bucks to do it, there are a few commercial products but they are expensive and are really for other applications.
