A portable but powerful compact heater than is ideal for heating garages, outbuildings, or home workshops has recently been introduced by Koehring, Atomaster Div., Bowling Green, Ky. 42101. Called the M-30 Reddy Heater, it is a miniature oil burner that is designed to run on kerosene when plugged into a standard household electric outlet.

The heater consumes only about one-quarter gallon per hour while delivering up to 30,000 BTUs - more than enough to warm an average two-car garage or sizable home workshop and storage area at a cost of approximately 11 cents per hour. An efficient, high-capacity blower circulates clean hot air that is filtered as it flows through the unit, and a solid-state flame-out protection device automiatically shuts everything down when there is any kind of malfunction. A smaller version of larger commercial units that have been in use for years on building sites and factories, the M-30 is priced at $126.95, with combusion chamber and other key components made of stainless steel. Plastic Storm Window

To stop drafts and help minimize heat loss through windows, the Deflecto Corp., 7035 East 86th St., Indianapolis, Ind. 46250, has put out a Model ES-65 Storm Window Kit which enables the homeowner or apartment dweller to install his own storm window inside or outside the existing window. Priced at just under $9, and containing enough material for openings up to 4 feet by 6 feet, the kit includes 20 feet of a rigid vinyl molding that is fastened around the frame with nails provided. Twenty feet of plastic spline is also provided to hold the sheet of flexible clear plastic in place when pressed into the grooved molding. Once installed, the storm window is reusable year after year.Simply pull out the spline, roll up the plastic sheet and store both until next year. The grooved molding remains in place, and is scarcely noticeable. Pressurized Caulking

Do-it-yourselfers can now buy caulding compound in a pressurized can that eliminates the need for squeezing tubes or using a caulking gun. Called UM Vinyl Latex Caulk, it is suitable for use indoors or outdoors. It can be used for sealing around tubs and sinks, for grouting tile, or for sealing around windows and door frames. It will not bleed through when painted over and it is dispensed by simply pressing a plastic nozzle on the can sideways. Compressed air on the inside forces the compound out neatly, and any caulking that is left remains soft and workable. Made by Ultramotive Corp. Cushing Blvd., Bethel, Vt. 05032, UM Claulk sells for about $1.69 in an eight-ounce can. Home Repair Clinic

(Reader sending in question about home repair problems should address them to: Home Improvement Department, The New York Times, Times Square, New York, N.Y. 10036. Only those questions of general interest will be answered here.)

Q: I have a marble-topped coffee table which has numerous marks and rings where people have put glasses and plates down. Can you recommend anything that will take these stains out and make the marble shine again? R.B., Brooklyn, N.Y.

A: Most marble dealers and many department stores sell special cleaning compounds for marble. Unfortunately there is no one cleaner that works on all stains - it depends on what caused the stain. This means that you may have to try several different cleaning compounds or stain removers, so it may pay to buy one of the marble cleaning kits that come with several different materials and directions for using each. If the top is really bad, it may pay to take it off and bring it to a marble dealer who can then polish it professionally by machine. The shine is restored, and maintained by the application of a white paste wax - and this wax finished should be renewed at least twice a year.

Q: Our bathroom ceiling has developed numerous rust-colored spots, which are concentrated above the tub-shower area, although some spots are also on the upper part of the walls. The ceiling was last painted about two years ago with a latex enamel. We recently scoured these spots off and rinsed the area, but within two weeks the spots began to reappear. Do you know how we can get rid of them? S.R., Bronx, N.Y.

A: This type of brownish spot may be due to something leaching out from under the top coat of paint - usually some type of water-soluble resin in one of the previous coats of pain, or possibly a glue size that was once applied. Either way, the brownish resin is water soluble, as evidenced by the fact that it comes out mostly over the tub where steam rises and condenses, and by the fact that the spots can be washed off.

One permanent cure for this condition that usually works is to first wash the spots off again. When the surfaces are dry, apply one coat of a stain killer, such as Enamelac or BIN. Then paint over this with regular enamel. I would recommend an alkyd for further protection against leaching for beneath.

Q: My formica kitchen counter is about 12 years old and is white with gold flecks. The shiny surface is gone and the gold flecks are beginning to disappear, especially right next to the sink. I can feel the tiny pits where these flecks were. Other than that, the formica is in excellent condition. Can you recommend anything to resurface this counter? Mrs. E.J., Forest Hills, N.J.

A: If you want the same durability and finish as you originally had, then the only way to permanently resurface this counter is to apply new plastic laminate on top of the old. To do this you would have to sand the old surface down to dull it, then use contact cement to apply the new material. This might also mean taking out the sink and the moldings around the edges (if the counter has them now), then applying new moldings when the new material is on.

Since this is quite a job, you might prefer to paint the counter top with a two-part epoxy paint. This paint is very tough and will last for some time if you are careful about cutting on the surface, but it will still have to be renewed periodically.