Petra

There is a shortcut worth knowing about for shallow damage, and a warning that comes with it: a thin coat of clear varnish or clear nail polish fills fine scratches optically and restores gloss without removing any material.

Thinning it slightly gives a smoother, less obviously coated finish. On a decorative item it can look very good.

The warning is that it is not reversible in any easy way, and for something with sentimental or collectable value that matters. Polishing removes material and leaves original surface; coating adds a layer that will yellow eventually and that a future restorer has to strip.

So for a toy you love, polish. For a toy you are patching up for a shelf, the coating is quick and effective.

Candle wax is the obvious answer for your use case and it is cheap by the kilogram.

It melts easily, takes pigment or ordinary candle dye, sets opaque and solid, and pours around an insert without any trouble. Paraffin is the cheapest; soy wax is softer and slightly less prone to shrinking away from the glass.

Two techniques worth knowing:

• Pour warm, not hot. Very hot wax poured into cold glass can crack it, and it shrinks more as it cools, leaving a dip in the middle. Warming the jars first and pouring in two stages fixes both.
• You may not need to fill it. For something viewed from a distance, swirling a thin layer of wax around the inside of the jar so it coats the glass reads exactly the same as a full jar and uses a tenth of the material. This is the standard trick and it also keeps the weight down enormously, which matters if they are being carried.

That second point is probably the whole answer for a stage prop.

A different angle that people often miss: do the folding and cutting from the back.

Most black card is black on both sides, but a lot of craft card is coloured on one face only. If yours has a lighter reverse, print or trace on that side, cut from there, and score so the fold goes the right way. The finished model shows only the black face.

Even with card that is black throughout, working on the back means any stray pencil mark ends up inside the model. It is worth checking which side you actually need to be clean before deciding how hard the marking problem is.

It is a bad idea, and the reason is short: acrylic paint is water-based, and water interferes with epoxy curing.

Epoxy cures by a chemical reaction between resin and hardener in fixed proportions. Introducing water — and acrylic paint is largely water plus binder — can leave you with soft spots, a tacky surface, cloudiness, or in bad cases a batch that never fully hardens. Small amounts sometimes get away with it, which is exactly why the advice gets ignored until somebody loses a piece.

What you actually want is the pigment without the water: titanium dioxide powder, which is the standard white pigment used in almost everything, including the acrylic paint you were about to use. Adding the powder directly gives you the colour and leaves out the vehicle.

Very little is needed — start with a fraction of a percent by weight and add gradually. Pigment powders are far stronger than people expect.

Practical consequence for anyone mixing colourants: the model matters less than the specific pigments in front of you.

Two tubes both labelled magenta can absorb quite different parts of the spectrum, and a mix that works with one set will not with another. This is why experienced painters and dyers keep a mixing chart of their own materials rather than relying on a colour wheel.

The theory tells you why mixing darkens. Your own chart tells you what you will actually get.

One thing these are genuinely good at, and it is worth mentioning because it is not obvious: they are a decent backup rather than a primary kit.

Flat, nearly weightless, sits permanently in a wallet or a pack lid and is forgotten about. Anyone who fishes or hikes already carries better versions of every tool on it — but the better versions are in the bag they left behind, and this one is in their pocket.

That is the category it belongs to: redundant equipment you never intend to use.

If you want to stay with what you have, there is a workaround: tape the joint for structure and then cover it with a thin layer of paper and PVA glue.

A strip of ordinary paper, or a piece of tissue for a nearly invisible edge, glued over the taped seam gives you a paintable surface. It is the papier-mâché approach and it works on any joint regardless of what is underneath.

More steps than the gummed tape, and useful when the tape is already on and you would rather not rebuild.

One more option for corrugated specifically: for a really clean edge, mitre the joint and glue it with PVA, then reinforce from inside with tape where nobody will see it. The outside is then bare cardboard everywhere.

One thing to check if you go the carrier-sheet route: the adhesive strip must lead into the printer, not trail behind it.

A note fed adhesive-edge-last has its free edge lifting as it goes through the rollers, which is how it catches. Adhesive edge first, firmly stuck down, and it usually goes through cleanly.

That said, I lost an afternoon to a note wrapped around a roller once and would not do it again for a daily task.