This addresses the perfect solution for silnylon. http://jwbasecamp.com/Articles/Silnylon1/index.html
Or if you are wanting to continue with instruction that specifically discuss PU coatings there is this:
The processes are identical, one just cites one specific kind of material while the other offers a more general application base.
In case of page deletions here is a brief summary of the information:
Shelters made out of cuben fabric never need re-coating. All other fabrics do have a coating which will degrade over their lifespan, mainly due to rubbing, the degrading effect of UV light from the sun and storing it wet. Shelter fabrics usually have either one of those two coatings: a silicone coating or a PU-coating (polyurethane). Tent floors usually have a PU-coating while tent flysheets usually have a silicone coating. However, a PU-coating is still used by some manufacturers on their tent flysheets. It is important to know which kind of coating has been applied to your shelter as each of these two kind of coatings need another product and solvent for re-coating. Some tents like for example those of MSR even have a different coating on each side of the fabric of the tent fly, a silicone coating on the outside and a PU-coating along the inside. If you’re not sure about the type of coating, try to contact the manufacturer.Coatings based on silicone are the easiest to renew. In case of a PU-coating it is more difficult to find the right product and solvent.
Ok, what do you need?
These are the necessary items for a silicone coating:
A tube of silicone: transparent and odorless (without additives)
A silicone syringe
A clean pot or jar
An accurate weighing scale (one gram accuracy recommended)
A paint brush
A pair of protective gloves (you’ll notice I didn’t wear gloves but you should know better)
For a PU-coating you’ll need an urethane based product and an appropriate solvent. One can also try the expensive option like Tent Sure Tent floor sealant from Mcnett or the Nikwax variant. Otherwise the method for a PU-coating will be rather similar as I’m going to explain for a silicone coating.
1.Pitch the tent in a well ventilated area, preferable outdoors(on a sunny day). If weather conditions cause a good bit of dust and debris to carry in the air wait, or move indoors. While minimal dust will not affect the quality of your application large particles (like the neighbor mowing) will affect adhesion.
2.Clean to prepare fabric for coating.
Make sure that fabric is completely dry before attempting to recoat.
Pour white spirits(15) and silicone(1) into a jar (weighing) the components so that there is a 15:1 ratio. As a general guideline try to using a 1.5g/1 sq. m estimate when purchasing materials. Consider each side of your tent/tarp an independent area if you are going to coat both sides.
Stir until spirits and silicone are dissolved.
4.Paint mixture onto fabric ensuring that no runs or drops gather on the fabric. Once dried touch the surface to determine if coating is thick enough. IF necessary apply another coat. Some minor discoloration may be noticed due to operator errors. (Who can perfectly and evenly paint that kind of surface?)
5.After you are confident the tent is 100% dry test the coating with a spritz of water. If the water beads and runs off you have succeeded. If not repeat steps 1-5 again.