Glue Joints between already painted parts

[Guest]

Consider two parts to be glued. The surfaces of both have been already painted. The surfaces to be glued are long and narrow - the usual thing - sometimes 0.5 mm wide. Please see the attachment (the part shown is 35mm wide and it is the glued-shut door of a 1:24 scale commercial vehicle).
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Scraping these to-be-glued surfaces after the many parts of a model have been primed then painted is very time-consuming. Someone once said that leaving them inscraped gives the joint the strength only of the paint/paint which is much lower than if the bare scraped surfaces are touching.

Obviously, for example, if a part sticks out like a handle which mighe be knocked off and it has a very small gluable surface, the paint should be scraped off for strength reasons.

For joints that do not need to be especially strong, like the above door which will be glued shut, is it necessary to scrape the glued surfaces?

Thanks.

[Gern]

I always try to get at least one clean area for glue. It's always possible your part could come adrift if you have to mask it and then remove your masking medium, or have to sand it for the outer paint colour. For the part you've shown, I'd use just the tip of a small square or rectangular file to scrape off maybe a 1/4" of paint along the join on at least two of the sides.

If there are large parts that need to be painted or primed before assembly, I sometimes use very thin strips of masking tape to cover the glue areas before painting.

Having said that, I've noticed that some glues will dissolve the paint anyway - hence the need to be careful when adding all those bits you leave off during painting - like canopies and aerials etc. Too much glue will leave a mark at the join.

[Guest]

Steve, if you buy some size 15 swann -morton blades, they are perfect for scraping such surfaces, even really narrow ones, due to the tip of the curved blade being at the perfect angle as you are holding it! Failing that, I use masking fluid - so easy, if you use cheap Boldmere brushes, rinsing them well between each brush stroke, sounds more of a shag than it is, but is very easy to get accurate results!

[Ian M]

When I am forced to paint first then glue, on the easy bits, I scrape the paint off with a blade or sand it off if it is flat and even. Otherwise a length of 10 mm masking tape sliced up into 1 mm strips goes a long way....

[Guest]

If it isn't load bearing then I use superglue to bond painted parts and, so far, I have not had issues. When building the motorbikes, I finish painting loads of parts even to the varnish stage then complete the final assembly with superglue applied with an applicator or cocktail stick. Using a standard glue like Tamiya extra thin inevitably damages the paint.