A word on using restorers: most glass restorers in the US are self-taught hobbyists and not formally trained. We recommend asking them about their professional experience and credentials.
Virtually all restorers do work "at your risk" meaning that once you have handed the item over to them to perform the requested work, they will not be liable for any damage that may result. This risk is a necessity as it is not possible to insure glass or other extremely fragile items while elective work is being done on them. If the item should break or be otherwise damaged the restorer is not bound to cover your cost for the item and most household or other insurance policies will not cover the loss. Reputable restorers will have a policy of not billing a client for work when an item is further damaged by them. Ask about such a policy in advance.
Inspect all glass closely before giving it to a restorer, note any pre-existing defects or condition / execution issues. Chords or large bubbles indicate weaknesses and make the glass susceptible to breakage in cleaning.
Be sure to tell your restorer that Blenko is particularly "soft" (porous) soda-lime glass.
When working on rims, many restorers will want to finish the rim in the "best" (standard) way; that is with a bevel, which does not chip as easily. Bevels are inappropriate for almost all Blenko. You must specify that you want to maintain the existing "fire-polished" rim look.
For bowls, plates or exterior surface stains most glass restorers can polish these off with conventional polishing methods.
For internal stains or sickness on vessels, "tumbling" will be necessary.
Tumbling involves filling the item with an abrasive material (such as lead filings, bb pellets, felt, etc). The top of the item is then sealed and it is inserted horizontally into a tube and secured in place. The tube, lying horizontally is then rotated on a machine for an extended period (varying from hours to weeks). The duration depends on both the degree of staining and the type of glass.
WARNING: tumbling is not a problem-free process. A not uncommon by-product of tumbling is variously known as "rippling" or "the orange peel effect" wherein the inside of the bottle will develop a disfiguring texture (like rippled water or the surface of an orange). The risk of this is generally greater than many assume. Be sure to ask any tumbler in advance about this problem and what their payment policy is if rippling should occur.