Is the sentence "just to help me if I'd paint" grammatically correct?

Here is what Cambridge says on if with will/would (as 'd is clearly the verb contraction of would):

Will and would can be used in conditional clauses, either with the meaning of ‘being willing to do something’, or to refer to later results:

If Clare will meet us at the airport, it will save us a lot of time. (if Clare is willing to meet us)

If you would all stop shouting, I will try and explain the situation!

If you look up the meanings of would, you'll find that it can also

express willingness or consent (OxfordL)

Cambridge also points out that

We use would to talk about willingness in past time situations.

So your sentence could be read as

He said he'd come back every May, just to help me if I would be willing to paint.