Q: Do Tarptents need to be seam-sealed?
A: Yes, it's a good idea to seal the the seams along the rear arc and the pullouts to protect the stitching. The ridgeline seam can also be sealed but has proven to be extremely water resistant without sealant. Silicone is the only material that will stick to the fabric. Urethane sealer will flake off. An inexpensive product that works well is GE Silicone II clear sealer/glue. It is available as a squeeze tube in most US hardware stores or in the Tarptent Store. Set up the tent ouside in a in a well-ventilated location. Mix about a tablespoon of silicone with a couple of tablespoons of mineral spirits(paint thinner) and then apply the solution with a small foam brush. Add more mineral spirits if the solution gets too thick. Avoid skin contact and breathing fumes.
Use those 3/4" throw away foam brushes (40 cents), stir the mix until it has an enamel paint consistency. the advantage of this degree of liquidness is that the solution will penetrate the fabric making a much better seal, -- put the tent out for 3 days, on second day, do any touch up, as sometimes the fabric absorbption leaves "room" for more to be "foam painted" on the second day.
I did my tents this week. A good seal will hold for at least 2 years if not more.
The pre-mixed sealants at REI or a sporting good store are complete junk for the reason they are too thick and tend to only stick to the outside of the fabric rather than penetrating the fabric, even though the ingredients are probably the same.
It is important you add enough mineral spirits to get the liquidness of a paint.
ps. Because the mix is very liquid, when you seam seal from the top (outside), the sealant will permeate and penetrate through the fabric, so it is sealed all the way through the fabric. However, double check the sealing the next day, this permeation can cause the surface you seam sealed to look "not enough" the next day, because of that dispersion throughout the fabric and you will see in about 10 percent of the spots that you should have used more, which you add day 2.