CLeaning stone building needs careful study

Q: The masonry joints in the facade of our half-century-old limestone church leak. We've heard from three masonry companies, one for only a partial job of cleaning and sealing the joints. The other two suggest: (1) chemically clean the entire building, repoint where required, ''stripe'' the remainder of the joint, apply a clear sealer, and recaulk all windows with a silicon sealant at a cost of $18,000; or (2) pressure-clean the entire edifice with water, rake out the old joints to a depth of one-half inch, repoint all joints and recaulk windows with a silicon sealant, and apply Ombrella 9100-brand clear sealer at a cost of Timothy Werbstein Miami, Fla. A: This question has so many angles that we wrote to the Indiana Limestone Institute of America, Stone City Bank Building, Suite 400, Bedford, Ind. 47421, for advice.

(The institute replied in a two-page letter which we sent to Mr. Werbstein and is far too long to elaborate in this column.

(The letter covers the mortar repointing and ''striping,'' chemical and pressure-cleaning caution as to the use of acid-base cleaners vs. alkaline, minimum expectation of the best results from single cleaning methods, the mistaking of fungus for dirt, care in joint sealant usage, longevity of compacted mortar joints, caution in the use of sealers, usage of water repellents that ''breathe,'' the trapping of moisture in masonry, necessity for skilled applicators, etc.)

''Higher costs do not necessarily assure better results,'' says the institute , ''but in building maintenance, as in most other labor-intensive processes, you tend to get what you pay for.''

I'd suggest that you carefully study the limestone institute's comments. I'd also get in touch with the Building Stone Institute, 420 Lexington Avenue, New York, N.Y. 10017, (212) 490-2530, and the Department of the Interior, Historic Preservation Section, Washington, D.C., for additional comments.

Only then would I sort out, step by step, the procedures and make up my mind.

I'd then ask for competitive bids from two or more reliable masonry contractors for the specified work.