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Before we paint any pre-painted surface, we gotta make sure that the surface
is sound. In other words, that the coating that's on the surface is firmly adhered
to the surface.
If we don't check,
then what could happen is when we put the next coating on
it could soften that coating and it could begin to peel.
The way to check that is to do a cross hatch test.
Tools that we need to do that
is basically
a simple stanley knife
and some good strong masking tape.
TIM: We first start off by using a sharp knife
and score the surface in a cross hatch design. We then take a length of tape
and apply over the cross hatch,
ensuring that we cover the entire cross hatch
and press down firmly.
Then we peel the tape back over itself,
keeping our hands close to the surface and remove the tape in a quick action.
And finally, inspect the tape
and note how much paint
has come away.
So from the test that we've just conducted, we can see that the surface
is in very, very good condition, and is classified as what we call sound,
so we can just go straight ahead and prepare it as normal and start painting.
Okay, now let's have a look at a surface that is classified as unsound.
TIM: Okay, so... here we have the surface.
We do the same as before -
a cross-hatch.
So we take our masking tape, as before.
Nice strong masking tape.
Place it over the surface
and press down nice and firm.
TIM: So now we
grab the tape
and pull it over itself in a fast motion,
and already
we can start to see how
the top coating has started to come away already,
very, very easily.
Okay so we just have a look at now the back of the tape,
and you can definitely see there
the coating has
definitely come away.
And in this scenario
what we'll have to do is probably do it a couple of times in some other areas as well,
just to make sure that it's just not in that particular spot. And if we find that
other areas are doing the same,
then really we classify this as a unsound surface.
And we've got two options:
either we can strip the entire surface back
and start afresh, or
we put a coat of oil based sealer over the top to seal it back nicely
and then progress with the successive coats.