soaking vs brushing to apply silicone to an old tent

I think you want to paint it, to get good contact between silicone/mineral spirits and nylon.

Sort of like painting a house – sometimes spray paint on then brush it to get good contact

I've painted silnylon pack which worked good, but tent is a lot bigger area

One way to search is to google "slicone tent site:backpackinglight.com" (or whatever)

Pretty sure the exterior side of the Fling is silnylon. If you're concerned about waterproofing, dilute a bunch of 100% silicone in mineral spirits (the mixture should be thin and semi-translucent) and brush it on with a foam brush. It doesn't take much to seal seams or revitalize a fly which might have seen a bit too much UV.

Mix a 5 : 1 ratio of ODORLESS mineral spirits to GE clear silicone caulk (by volume).

** SHAKE MIX JAR WELL EVERY 5 MINUTES
1.>Mix in a pint (or quart jar (depending on treatment area size) with screw-on cap and shake very well.
2.>lay fly or floor flat on small table (2' X 3" is ideal) with NO wrinkles or material beneath
3.>Pour part of the mix (1/2 cup) on small paint roller tray and use a narrow (3 ") paint roller with very low nap.
4.>Roll on about 2 sq. ft. at a time
5.>Wipe excess off with a blue paper "shop towel". These towels will not leave behind paper bits. *(This step is very important to allow for complete drying.)
6.>Continue on to the next area. Overlapping is OK since you are wiping off excess anyway.

Repeat this procedure for floor (only) if you feel a second coat is necessary.
Clean up with odorless mineral spirits.

BTW, do NOT "dip" entire tent fly of floor in a mix. It would be unmanageable and hugely and unnecessarily expensive.

I read specs that mentioned both PU and silicone coatings on the fly. McNett makes some products for resealing PU coated tents.

A call to MSR customer service may get you a new tent :)

I have seen two types of odorless mineral spirits

1 – looks identical to regular mineral spirits. They must add small amount of something that reduces odor. This works to dilute silicone

2 – looks watery. This does not work to dilute silicone.

The vendor websites say the canopy of this tent is "polyurethane/silicone" coated.
Coated with PU on one side and sil on the other sounds logical, but that is not the case with the PU/SIL coated flies I've seen lately. Instead, they are coated with the same compound on both sides, and it behaves like PU, not sil.

You can find out by testing the side you want to recoat with a dab of Seam Grip or other PU sealer. If it peels off easily after drying, it is the silcoat we are used to, and you can choose one of the options posted above, or try a DWR spray noted below.

If not, it is behaving like a PU coat, and you could try a PU recoater like Tent Sure.

In either case, you might get good results from one of the DWR sprays like Camp Dry or Scotch Guard, that come in different formulations for silicone and PU coated fabrics. I've had good luck with them in rejuvenating a silnylon fly that was looking worn out. The only one that did not work well was the Atsko sil spray.