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I'm thinking i'd graft one of my mammillaria compressa pups as a backup. Wondering if the milky sap in it will stop it from taking to the stock. Any experience?

I have only grafted a few Mammillarias but in my experience the milky sap doesn't affect things one way or another.

I would not think so Elie if you can keep pressure on the scion until it takes. After all they can graft Euphorbia's and their sap is even worse.

For beginners you can only graft on like for like, meaning plants in the same family. You cannot graft Cacti on other Succulents or vice versa, it has to be Cacti on Cacti and Euphorbias on Euphorbias etc. Otherwise like many human organ transplants you get rejection.

http://www.teesside.bcss.org.uk/BCSS/CU ... afting.htm

A friend of mine uses wool rather than thread since it is not as liable to cut into the scion.

https://www.cactiguide.com/forum/viewtopic.php?t=27700

Thanks for the encouragement i'll try it soon.
Dave my favourite stock is opuntia ficus indica as it can really push the scion 365 days a year though it's a bit awkward to graft to. I don't use any thread. Instead i put a piece of folded paper tissue and then add pressure over it with hypoallergenic tape. It sticks very well on the opuntia pads. I only added the paper tissue because the tape would stick to the scion and either break the joint or peel off the spines and apex