After you have prepared your window frame by sanding, scraping off loose putty, and paint and filling cracks and holes, dust everything thoroughly with a dusting brush. A crucial aspect of the preparation that many people omit is the priming of the rebate. It is necessary to prime any bare wood in the rebate before puttying because otherwise the wood will suck the oil out of the putty as it slowly dries and the putty can crack (and even fall out) as a result.
PREPARING THE PUTTY:
When you first open the tub of putty, the first thing you need to do is remove all of the putty from the tub and knead it. The oil in the putty often rises to the surface, and kneading helps to achieve a consistent texture. If the weather is cold, the heat from your hands will also help to make the putty more workable. After kneading, you can return the putty to the pot and remove small amounts as needed.
PUSHING THE PUTTY INTO THE REBATE
The easiest way of getting the putty into the rebate is to roll a small amount between both hands into a sausage shape and then push it into the rebate with both thumbs. You want to put more putty in the rebate than will be needed as you will trim off the excess in the next step.
CUTTING THROUGH THE PUTTY
Now you’ll need either a putty knife or a stiff scraper. I like to use a rigid 1” scraper. The aim is to cut through the putty. Don’t try to smooth it off as if you were using filler. As you push the putty knife or scraper through the putty, you should keep one of the corners of the blade against the glass and the flat side of the blade against the edge of the rebate as you slowly drag the knife/blade along. (See below). As you cut through the putty, you’ll make a line and the excess will stick to the glass above this line. You can simply peel this off with your hand.
After you have cut through the putty, take a dusting brush and lightly smooth the putty. This helps to push the putty into the rebate so that it doesn’t fall out and it feathers out any imperfections.
HOW LONG DOES PUTTY TAKE TO DRY BEFORE IT CAN BE PAINTED?
This depends on the weather conditions and how deep the rebate it, but usually you can paint putty once it has formed a skin and this usually takes a week but can take longer. There is a common misconception (which I think has been caused by people misreading the instructions on the backs of putty packaging) that you must leave putty for 28 days before painting. What these instructions tend to say is that you must paint the putty BEFORE 28 days. This is because once the putty has fully dried out, it becomes absorbent and if you leave it without painting it could crack.
Linseed oil putty is a lot easier to use than acrylic putty (a newer invention) but one of the limitations is that it can only be painted with an oil-based paint. If you try to use a water-based paint, will probably peel off. If you really want to use a water-based paint you will need to buy acrylic putty and a sealant gun. Be aware that the technique that is needed to use acrylic putty is completely different – and not very easy!
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