Basically, if you can't patch them individually there are only 2 real options left.
First, get a new tent. Every tent I have had has developed this over time unless used with a tarp underneath. Any roll on sealant that would work, would increase the weight about as much as a light weight tarp. Also, they tend to make the tent harder to roll up, and more annoying to deal with in general.
Second, Replace the bottom tarp. If buying a new tent is too expensive, you can replace the bottom panel. I have done this in a few tents before. It is not fun and if you do this, make sure you seal the new seam and keep an eye on it the next time out. There are companies that will do it for a fee but it often costs close to what a new tent costs, unless your manufacturer does this. Manufacturers are usually much cheaper.
As mentioned above, there are roll on sealant solutions, but in my experience they are not worth it.