If you are making a yeast starter, IMHO, I would use a blow off tube and keep it in place for all your fermentation. When I pull my blow off tube after 4 weeks to check gravity, then I will replace the blow off tube with an air lock until I keg. I never used Fermcap. I bought some to use when making a starter but changed the equipment I use instead. So I can’t say that it would eliminate the kraeusen. I would think that producing kraeusen is a vital part of the fermenting process and wouldn’t want to disturb it. I would think it is a build up of co2 that pushes it out when you have a blow up. A blow off tube is the way to go.
I bought a 5 cu.ft. Chest freezer and a three piece air lock. I thought the lower fermenting temp would prevent a blow up. WRONG!!! After the second day I had a big big mess. I then did what I should have done in the first place, I put the blow off tube in. I use a 1 inch diameter vinyl tube shoved in the 6.5 gallon carboy and don’t have a problem. You can try that for your set up.
