The corner of my rear bumper is chipped and cracked. I sanded it down, applied filler, undercoat and paint. It looks fine but I can see a little line where I masked off the area.
Are there any tips for avoiding this?
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Just for further elaboration, do you mean a line between your new coat of paint and the previous coat of paint? Is it possible you supply a picture please?
Over time, the original paint of a vehicle fades and oxidises. Barring extreme cases, this process is usually not noticeable unless there is a new coat of paint to compare it to.
The cut-off lines you describe could possibly be due to the contrast between the fresh coat against the faded original coat. This effect may be emphasized if your original coat of paint uses a clear top-coat and you did not apply a new top-coat to your working area. If your vehicle is of earlier make and model, it is possible that your vehicle does not have a clear top-coat at all, but rather just a very thick base coat.
To diminish (or completely remove) the contrasting effect, you will need to cut and polish the working area and the area surrounding it.
In this scenario, I recommend using a separate cutting and polishing compound as opposed to a mixture of the two, often seen in auto stores as 'cut 'n' polish'. New layers of paint and clear top-coat tend to be quite thick, and thus require a more abrasive compound to even out the differences between it and the original surrounding coat.
Cutting compound leaves behind a dulling or hazing effect on a coat of paint, which pure polishing compound (NOT a glaze) repairs, glossing it along the way.
You can polish either by hand or by machine. Hand polishing can be time-consuming and can often lead to defects such as holograms on your working surface. DA (Dual Action) polishers are quite paint-safe and do not carry the risk of damaging the working surface or burning through it.
In your case, I recommend hand-polishing. It may not be sensible to buy a machine polisher for just a spot-fix, as they do carry a >$100 price-tag.
You already got some good advice on buffing and polishing. Here are some tips to hiding paint lines and minimizing their noticeability:
I'm not an expert in this field, but when I have undertaken similar small DIY projects I tend to apply masking tape further out on the panel than I need to. I then stick newspaper to the tape and fold it to fit closer to the size of the repair. So long as you don't go mad with the paint this leaves a softer feathery edge as the paint will overspray a short way under the folded edge of the paper, leaving a gradient rather than a hard edge. This can then be wet & dried and polished smooth a lot easier.
It's great you have attempted to have a go yourself to repair your bumper, and sounds like you did a really good job for your first try....but the reason for the lip is poor masking technique, so just sanding, and repainting won't achieve much if you mask up again the same way....you will get another line. When you mask, roll the tape along it's length up and over itself so you don't leave a straight edge...it's a fuzzy line that's left and much easier to rub out. Furthermore, others have suggested using a buffer...DONT! Not unless you really know what you're doing and have a sound knowledge of these machines with much experience and you know you have applied enough paint to allow machine cutting. Trust me, a flexible block is indeed correct, as is 1000 grit wet/dry paper used wet...do by hand without too much pressure, let the paper do the work. When the lip is gone, compound n polish by hand, seperate compound and polish. A quick note:. With a machine, if you are cutting correctly, you will get a magnificent gloss, not a haze as has been suggested, but since the finish will be bare to the elements, polishing is required to seal that wet-look mirror gloss, then a machine wax to seal the lot....then a hand wax! if you have a dull haze after you think your finished cutting...you haven't....it will look ok after polishing, but a far cry from the crystal finish of a correctly cut job. There is a reason why spray painters spend 4yrs to complete a trade, then another 4yrs to become proficient, but having a go is still great to hear, that's why I've taken the time to steer you on the correct course so that one day you can lay down that trick custom paint job that blows people away. Talk to painters over 40, listen to them, you will learn much