When moving into a new old house, things can be done with enthusiasm that later seem like big mistakes.

Suppose you have a kitchen with an insulation problem. And not knowing any better, you panel the walls with inexpensive grooved hardboard, thinking that the three-quarter-inch air space behind the furred-out paneling will help. It can, but it doesn't look very authentic.

The look of old, rough plaster can be simulated on new drywall paritions, and it also works on hardboard panels. Here's how.

First, paint the panels with a flat white oil-base paint to cover a darker color underneath. Then trowel on a thin skin-coat of "silulated plaster," which is a thick mixture of joint compound-the kind you use on the seams in plaster-board partitions. Use the dry powder and mix it yourself to the desired consistency. (It costs about $4 per 25-pound bag.) You can also get it premixed, but it is more expensive this way and you have to work with it as it comes from the can.

One great thing about joint compound is that it stays moist and workable for a long time. If you make up a large batch., you can even keep it overnight by just covering it with a damp cloth.

The joint compound should be the consistency of thick mud so that it doesn't ooze off your trowel. Compound can be applied to the wall with a wide-bladed (6) inches or wider joint knife or a cement trowel that has some flexibility in the blade. Work about a two-foot-square area at a time.

A wide range of textures and effects can be obtained, from smooth to Spanish stucco. To create a realistic old plaster wall effect, try for a generally smooth appearance, with scratches, bumps and gentle waves here and there.

Frequent "cat tails" will be created by the edge of your trowel. Some of these should be left, because they can be very effective in giving the over-all antique effect. Build-up should be 1/8th to 3/16ths inch at the most.

You can go from almost nothing to a 1/8th-inch high spot in the same general area. Create this variable-thickness effect by sticking a glob of joint compound about the size of a fist onto the wall and then work it out: Left to right, up and down, with most of the finishing being done with the vertical strokes. Once you start, you'll find the strokes pretty quickly that will give the effect you want.

Any rough spots that you don't like once the material is dry can be sanded or smoothed over with a damp sponge.

When dry, the surface should be painted with a flat oil-base paint. Use a stiff, 4-inch brush, spreading the paint on rather generously if you only use one coat. It's not necessary to have complete paint coverage in all the little dents and valleys. In fact, it's better to have a few little skips here and there when you're using an off-white beige color. The occasional skips create subtle highlights in the surface and enhance the old appearance.