If you have a bay window in your home, chances are you use it as a sort of mini greenhouse. Most people do. A bay lets in lots of light, and its oversize sill or "seat" provides lots of space for your plants.

Trouble is, it doesn't take long for the dirt and water in the planters to go to work on the seat. Soon it's disfigured with stains, water marks and mildew. Here are some suggestions to solve those problems:

One of the best ideas I've seen is to have a shallow copper pan -- about two inches deep -- soldered up in the shape of the window seat. Then fill the pan with about an inch of crushed marble or other decorative stone. If any water leaks through, the pan will catch it. No damage will be done. In fact, it's not a bad idea to deliberatly wet the stone. This will help raise the humidity around your plants and improve their health.

Another way to help the window seat stand up is to cover it with vinyl flooring. Tiles are easiest to work with, but seamless vinyl will give a slightly neater job. If you use tiles, try to pick a size that will require the least amount of cutting. Many window seats are 18 inches deep, ideal for a double row of nine-inch tiles. If you used 12-inch tiles, you'd only have room for a row and a half.

If you decide on seamless vinyl, make a pattern out of cardboard cut to fit the seat. Then use it as a template to cut out the vinyl. This will help avoid mistakes that could ruin a whole sheet of flooring.

For a really elegant treatment, try ceramic tiles. This will require more work than using vinyl, but the results can be worth the effort, especially if you use some of the custom, handmade tiles produced in Europe.

As for vinyl tiles, try to pick a size that fits your window seat. Three rows of six inches work in well with an 18-inch seat. Lay out all the tiles dry (without mastic) to make sure you get a good fit. You will have to make angle cuts on the tiles along the ends of the seat. The tile shop can loan or rent you a tile cutter for this part of the job.

If you have to make any irregular cuts to fit around pieces of window framing, use a tungsten carbide rod saw in a hacksaw frame. This type of saw -- Remington's "Grit Edge," for example -- is sold in hardware stores and will zip through the tile with ease.

When you are satisfied with the fit of your tiles, cement them in place with tile mastic. Ceramic tiles require grout to seal the seams between tiles. Some tiles are made with "lugs" on their edges to assure proper spacing and produce uniform seams between tiles. If the tiles you use have no lugs, make your own spacers out of scraps of hardboard, and place them between the tiles. Remove them once the mastic hardens.

After the mastic has set, apply the grout according to label directions. Remember, grout comes in a variety of colors as well as the usual white. Using one of the colored grouts can add a lot to the appearance of the job. After the grout has set up, protect it with a silicone type grout-sealer. This will keep it from soaking up water and possibly becoming stained. After all, that's the problem you set out to eliminate in the first place.