Cannabis applied to the skin rather than taken into the body: balms, creams, oils and patches, what the skin will and will not let through, and which of the claims printed on a jar have evidence behind them.
The useful starting fact is that intact skin is a barrier built to keep lipophilic molecules out. A cream rubbed on a joint is acting locally if it acts at all, which is why topical products do not produce intoxication and also why they are a poor route for anything needing a systemic effect. Transdermal patches are a different product category built specifically to defeat that barrier, and should not be discussed as if they were the same thing.
Two points recur. The carrier and the concentration matter more than the cannabinoid name on the front label, and many products sold as topicals carry too little of anything to test. And a skin condition that is getting worse is a dermatology question first: a balm that soothes is not a treatment for an infection or an autoimmune process.
Where to start
Related elsewhere on the forum: Routes of Administration for Medical Cannabis in Medical Cannabis.