Andy, if you have the floats out of the water, fill that compartment with water and check where it is leaking.
1. ensure the outside of the float is xompletely dry so you know when you see water, it is coming out of a leaky area.
2. check for loose or worn rivet heads
3. check any seam
4. check the keel area...sometimes the bottom skin will split at the bend from the keel-flat to the side-skin area.
5. you may need to drill out rivets, open a seam, reef out old dry sealant (what vintage are your floats?), apply new sealant, and re-rivet
A. use 3M 5200 Marine sealant FAST. It is THE standard. There are similar products.
B. Alcohol inhibits the cure of any polyurethane sealant......it says that right on the tube of 5200. I will sometimes clean with alcohol, but always let it evaporate before applying the sealant. As to AKTango's trick, I will not ever try it. (Sorry George, maybe I should not read the instructions
)
C. wear gloves, and change them often as it becomes difficult to work with sticky fingers.
D.clean up excess sealant with Alcohol or Stoddard solvent on a rag. (rags are better than paper towels by far).
E. Clean any scum from the surface you intend to apply sealant to. I use warm soapy water, then sponge and wipe dry.
F. Polyurethane sealants are moisture loving and cure fine in a moist environment.
G. If you find the leak with the float in the water, apply the sealant and then fill the float compartment to the waterline/lake-level to equalize inside and outside pressure to ensure the sealant stays in place during cure.
a. Splash Zone is a wonderful repair/sealant, that is often used as a temporary repair both externally and internally. It can be difficult to remove, sometimes requiring grinding to remove. Any floatplane guy should have it (and also some 5200 FAST)
b. PRC/flamemaster two-part is a pain in the arse. (fiddly mixing ratio's, expensive, product goes out of date, super-stinky. However, it is supersticky!)
c. If you don't know what you are doing with leaky floats, you can learn.
d. as SB said, how many pumps? how many are you willing to live with? Also, is this a hatch-seal issue rather than a leaker?
Good Luck, you drew about the least fun thing to fix on an airplane.