I have a rain fly that has some of the seam tape coming off. It’s still attached on a lot of the seams in some places. I have seam sealant and some waterproof spray for tents. Should I remove all of the seam tape and use the sealant or save what seam tape I can and seal around and on top of the remaining pieces? Should I seal both side if I remove or salvage the seam tape?
I would remove any seam tape that is not stuck and seam seal those seams.
Where the seam tape is lifting at the edges here and there but clearly still seals the seam i would leave it alone but in small areas where it is partially lifting away from the seam but otherwise stuck I would use a little seam sealer to stick it back down.
But probably I would just buy a new tent. Tent fabrics age out and can become weak even though they still look OK. All that work could end it tears so only invest the time if you have nothing better to do.
While seam tape and seam sealer serve similar purposes, they work differently. The seam tape forms a barrier inside the seam, and prevents water from going any further. Seam sealer basically just fills in the holes around the thread. It can be sufficient without seam tape, but if there's even a tiny gap somewhere, you'll have a leak.
I would remove all the old seam tape that isn't completely secure, use the seam sealer, and replace the seam tape if you want the tent to be "as good as new". (I'm assuming it has been stored properly and the fabric hasn't deteriorated.)
Hmmm you’re going to have to seam seal the heck of your tent, probably every where,
make your own with clear silicon mixed with paint thinner into a slurry then brush on the outside seams, then I recommend spraying the whole thing with a can of silicon sealer from Walmart