Basically Formalin is cytotoxic and with its strong affinity for water will enter the sweat gland and from my understanding denature the cell walls. Regrowth occurs eventually.
I normally apply formalin for treatment of pitting keratoloysis which, in my opinion, should be the main stay of treatment for all cases.
It is applied once daily at 10% concentration for 1 week. Depending on results the patient can continue on for a further week or two if necessary. No doubt the condition will recur but probably not for another 6 months. Hence it is repeated as necessary.
Formaldehyde can be quite irritating to the skin. This varies from the type of skin it is applied to, to the genetics of the individual. Axillae I have never tried it on but I assume it would be more irritating . I would suggest daily applications at 3% for a week and then see how it goes. If it does nothing then try twice daily for a week. If still no significant irritation but not enough effect then bump it up to 10% and try the same process again.
Don’t need to wipe off just let it evaporate. Just apply with gauze or cotton wool. Tried to avoid regular contact with your fingers as it will dry them out.