I have an older well with a shallow well jet pump. About once a week I have to jam a screwdriver through the jet orifice access to knock out a piece of small scale. Is there a filter available to install with a large enough mesh not to hinder pump operation. I have a 3/4" line to pump from check valve.

Wouldn't hurt to give it a try. The filters like you are referring to are fairly inexpensive (fifty bucks or so at Lowe's, for instance). It is generally recommended that filters go after the pressure tank, but if it works, then hooray for you.

I think this is what you are referring to: Shop Whirlpool Valve-In-Head Clear with Sediment Filter at Lowes.com (http://www.lowes.com/ProductDisplay?partNumber=89376-59019-WHKF-DWHV&langId=-1&storeId=10151&productId=1201343&catalogId=10051&cmRelshp=req&rel=nofollow&cId=PDIO1)

knock out a piece of small scale
That implies build up of mineral deposits. Please describe.