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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.

  • Hi. Yes I mean there is a line between the new paint and old. The paint colour looks like it matches but there is almost a raised lip where the new paint is. Over time this became even more noticeable as fragments of dust highlighted the rise. – MeltingDog May 8 '16 at 22:52
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    To remove the raised lip: wet sand with a flexible block starting with 1000 grit. Try to sand the lip down to where it no longer catches a fingernail. Then move up to 2000 and 3000 grit papers, then cutting compounds, and finally polishing compounds. If it's a small area, you can cut/polish by hand. If larger area, see if you can rent a buffer or invest in a cheaper "consumer grade" buffer for $150 or so. If you sand through clear - STOP! You don't want to sand through the color! Just spray a little more clear over the area and you can sand some more. Eventually it will become smooth. – CBRF23 May 9 '16 at 23:13
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You already got some good advice on buffing and polishing. Here are some tips to hiding paint lines and minimizing their noticeability:

  • Always provide the paint code and the VIN when you have paint mixed. There are almost always variations in batch and manufacturers make slight changes to color formulas over the years, so paint code is not always enough by itself. This can make a HUGE difference.
  • Blend panels on the same plane; don't end at body gaps. Any change in color is more noticeable when it's on the same plane - e.g. a fender and a door are on the same plane (while a hood, trunk, or bumper are typically on perpendicular planes from a fender). When you paint a fender, it's often necessary to blend into the door, so the panel-gap does not contrast and exasperate the slight color variation. Shoot a little color (base coat) 6-12" past the panel gap and then clear another 12" or so beyond that (so clear would go 18-24" into door in that example).
  • Paint up to body-lines/features and use "soft" line taping techniques. Use the car's natural lines to your advantage for hiding a paint line; the natural shifts in light will make the paint line much less obvious. A good technique for softening paint lines is to take your tape and put it down so that its over (covering) the feature line and then roll the tape back to make a tube (like a curling wave that a surfer would ride inside) which the bottom of the tube (sticky part of tape) is tangent the feature line. This will soften the edge of your paint line and will help it to be less noticeable.
  • Use "soft" tape lines and an "expanding over-lapping area" technique for spot painting. While it's better to take the paint up to a feature line if you can, sometimes that's just not practical. The best way I've found to minimize a tape line is to combine soft taping with a technique of limiting and expanding the area at each stage (primer, color, clear). Staring with primer, you want to soft-tape off an area that is 1-2" larger than your bodywork area (it can be a square, doesn't have to follow bodywork shape - just has to be larger than the largest area of bodywork). After primer drys, block your primer out with tape in place, then remove that tape and soft tape off a slightly larger area (offset the line so it's 3-6" larger all the way around). Shoot your color (base coat) within this area and then remove tape. Now once again you will soft-tape off an expanded area, 6-12" larger than the color area, and this is where you'll spray clear. Finally, once dry, remove tape and then wet-sand and buff to blend. This technique helps to limit overspray and contain the blend to a smaller area. The technique of overlapping areas helps to mask color variations from base over primer vs base over color, and helps blend old and new clears to make all the transitions less noticeable. This also will help you avoid paint building up too thick and having a hard raised edge from the tape line (like you described in the comments on another answer to this question).
  • Try to match factory orange peel as best you can! Avoid wet-sanding and buffing by shooting paint that matches the factory finish :) A tip that seems to help a lot here is to add just a tiny bit of base-maker to your clear. It helps thin the clear just enough to sag and spread a little more, so you get that larger orange peel typical of the factory finish, as opposed to that real tight orange-peel indicative of most re-sprayed panels. If you get a bad match in finish textures, you may have to do a lot of sanding and buffing to make the repair work blend with the rest of the car. If you can match the factory finish, you're done. Maybe you might run over the area with a polisher for good measure, but that's about it. No need to sand or cut.
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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

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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.

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