Filling phone extension back box hole

[Jeff Layman]

A BT rewire has made an extension socket in the kitchen redundant. I've
removed the back box, but am wondering about the best way to fill the
hole before repainting. The hole is about 70 x 70 x 25mm.

I've got enough polyfilla, but it would take several layers allowing it
to dry in between. Or I could partially fill the hole with mortar and
let that dry before using filler. I've even considered putting the back
box in back-to-front and using a thin layer (2 - 3 mm) of filler over
it. Would the thin layer be strong enough? Could something else like
expanded polystyrene be used to partially fill the hole before using filler?

[John R Walliker]

Post by Jeff Layman
A BT rewire has made an extension socket in the kitchen redundant. I've
removed the back box, but am wondering about the best way to fill the
hole before repainting. The hole is about 70 x 70 x 25mm.
I've got enough polyfilla, but it would take several layers allowing it
to dry in between. Or I could partially fill the hole with mortar and
let that dry before using filler. I've even considered putting the back
box in back-to-front and using a thin layer (2 - 3 mm) of filler over
it. Would the thin layer be strong enough? Could something else like
expanded polystyrene be used to partially fill the hole before using filler?

A dry sand and cement mix should be fine, but bulking it up
with pieces of broken brick would be even better at avoiding
shrinkage.
John

[alan_m]

Post by Jeff Layman
A BT rewire has made an extension socket in the kitchen redundant. I've
removed the back box, but am wondering about the best way to fill the
hole before repainting. The hole is about 70 x 70 x 25mm.
I've got enough polyfilla, but it would take several layers allowing it
to dry in between. Or I could partially fill the hole with mortar and
let that dry before using filler. I've even considered putting the back
box in back-to-front and using a thin layer (2 - 3 mm) of filler over
it. Would the thin layer be strong enough? Could something else like
expanded polystyrene be used to partially fill the hole before using filler?

Irrespective of what you use its not going to be a one off application.

To avoid the possibility of cracking, especially around the perimeter of
the hole, seal it with a 50:50 PVA:water mix and allow to dry. Unsealed
Old brick or plaster surrounding the infill will suck the water/moisture
out of whatever you use and its liable to crack at that point during the
drying process.

A thin layer over a reversed back-box in unlikely to be satisfactory.

As suggested in another answer a bit of brick or stones or similar to
bulk out the infill means that you can use much less filler.

You could use mortar or your polyfiller to secure your infill -
whichever is more convenient, handy, cheaper. Make sure that the infill
is a few mm below the finished surface. Let it dry.

Even when you fill flush the filler may shrink back slightly when it
dries requiring another light skim over the top. It helps if your trowel
you are using for the final layers is wider than the hole you are filling.

[No mail]

Post by Jeff Layman
A BT rewire has made an extension socket in the kitchen redundant. I've
removed the back box, but am wondering about the best way to fill the
hole before repainting. The hole is about 70 x 70 x 25mm.
I've got enough polyfilla, but it would take several layers allowing it
to dry in between. Or I could partially fill the hole with mortar and
let that dry before using filler. I've even considered putting the back
box in back-to-front and using a thin layer (2 - 3 mm) of filler over
it. Would the thin layer be strong enough? Could something else like
expanded polystyrene be used to partially fill the hole before using filler?

- Paint inside of hole with dilute PVA.
- When PVA is dry (about an hour) fill hole with bonding plaster (may be
called undercoat plaster in the sheds), leave surface slightly (about a
mm) below wall surface.
- Next day (or later) use your polyfilla to fill the remaining shallow
depression, get it as flat as you can.
- When polyfilla is dry (next day?) sand it flat by wrapping 120g
sandpaper around a piece of wood that's long enough to bridge across the
hole.
- Repeat the last 2 stages if the result isn't as good as you want.
- paint with diluted finish paint, then a couple of undiluted coats.

[The Natural Philosopher]

Post by Jeff Layman
A BT rewire has made an extension socket in the kitchen redundant. I've
removed the back box, but am wondering about the best way to fill the
hole before repainting. The hole is about 70 x 70 x 25mm.
I've got enough polyfilla, but it would take several layers allowing it
to dry in between. Or I could partially fill the hole with mortar and
let that dry before using filler. I've even considered putting the back
box in back-to-front and using a thin layer (2 - 3 mm) of filler over
it. Would the thin layer be strong enough? Could something else like
expanded polystyrene be used to partially fill the hole before using filler?

I have many methods
E.g. (if you have some handy) use a piece of plasterboard wedged in the
hole below the surface of the finished wall and skim. There are many
other ways to do the wedging.
Simply get ANYTHING that will support the first bodge coat of plaster -
scrunched newspaper newspaper, a blown up balloon, used condoms,
expanding foam, and old bit of expanded foam packaging, or PU insulating
foam.

All it has to do is support the plaster till it sets.
And don't use polyfilla - its way quicker to use several coats of fast
setting plaster.