What to add to seam sealer to make it more fluid?

Mineral spirits, yes.

Odorless mineral spirits from an art supply store.

Tarptent’s Seam Sealing Video

try it first, let dry, make sure the seam sealer is good and you have the right mineral spirits

Vinegar

Never heard of using vinegar before.  I’ve sealed quite a few shelters using the “Tarptent method”.  Works beautifully.

I don’t like to seam seal so was delighted to find that my Big Sky tent doesn’t require it.

I have no idea how Big Sky does it but I ‘ve had the tent for several years and it has never leaked.

Daryl, you might remember when Golite made the same claims about their shangri-la shelters…that they needed no seam sealing because they used a thread that expanded when wet to seal the seam. Mine never leaked but nobody ever really found what thread it was.

I never seal seams on mids.  They are at a steep angle and any leakage just flows down the seam.

I don’t think there’s a need to seal mids

I’m going to have to try vinegar, but a bit skeptical

When silicone cures, it gives off a vinegar smell – acetic acid

https://handymansworld.net/how-to-thin-silicone-caulk/

From Dan Y’s link,

4 Ways to Thin Silicone Caulk
Alright, so now that we have covered all of the pros and cons of thinning down silicone caulking, let’s take a look at the various ways of actually doing so.

#1: Using Mineral Spirits
In case you did not know, mineral spirits are a clear liquid that is derived from petroleum. Generally speaking, mineral spirits are used in painting as a paint thinner and paint dissolver, and are used as a replacement for vegetable turpentine.
You can use mineral spirits to thin out silicone caulking that you are going to use in the future, and you can also use mineral spirits to dissolve old caulking that you want to remove. Although this method can be used for silicone caulking that is yet to be applied, it is best used for old silicone caulking removal.

#2: Using Vinegar
If you plan on thinning out silicone caulking for use, then what you want to do is to mix it with vinegar. It may take a while to get the proportions right, but start out with something like ten parts silicone caulking and one part vinegar and then go from there.
If you plan on using vinegar to remove old silicone caulking and simply for a few cups of it in a bucket, get a rough scrubber, dip the scrubber in the bucket of vinegar, and then get that scrubbing.
Vinegar will definitely cause quite the smell, but it is the most natural method to consider.

#3: Using Silicone Solvent
Yet another thing that you can do to fit down silicone caulking is to use a silicone solvent. Now, this is best done if you plan on removing old silicone caulking, not for thinning out new caulk that is yet to be used, although it is definitely a possibility.
Silicone solvent is very strong, so you do need to be careful in terms of how much of the solvent you add. Be sure to wear gloves and eye protection when working with this stuff!

#4: Using Alcohol
The other substance that you can use to thin out silicone is some good old alcohol. Yes, here we were talking about pure rubbing alcohol, not a bottle of Jack Daniels. Simply keep adding alcohol into the silicone caulking until it is thin enough to meet your requirements.
Yes, rubbing alcohol can also be used for silicone removal too.