How can I avoid sap seep in my woodworking projects?
Yes it's normal to hit pitch pockets in douglas fir. You can remove it by wiping it with mineral spirits but that's only going to clear the surface and more will weep out; you could be fighting it for years. Because of the pitch (among other reasons), doug fir is not the best choice for furniture. If the wood is prepped correctly by sufficient kiln drying it is fine, but sufficient is often an unknown quantity. You are better off choosing hardwoods like oak or maple ( among others).

I do not know how mineral spirits will interact with your finish. Also mineral spirits can be harsh and if you are sensitive to chemicals you can look into odorless mineral spirits which might help.