What is the mistake a rank amateur at this is most likely to make?
Honestly.. it's attempting to do it themselves. You mention:
I am an OCD type
I can pretty much guarantee that the result of doing a touch up will be just as noticeable (if not more so) than the original mark. As a fellow OCD type I can certainly sympathize and it's because of this that I implore you to let a professional handle this.
To get it looking substantially better than it does now you would need to get a great paint match, be very, very, careful when applying the paint and be prepared to spend the time machine polishing or better yet wet sanding the touch-up area. But these are not skills you just do "right" first time.
Getting paint and a fine brush on line and dabbling it myself is far, far less hassle.
It's really not.
This won't take hours.
It really will. And here's why;
Modern automotive paint is what we call "two stage" paint - on top of the (usually primer coated) body work there is a first stage of the color (the actual paint) and on top of that is a layer of lacquer (aka clear coat). From your description of the ding you've gone through both of these layers (assuming the "substrate" you refer to is the primer underneath) and you need to replace both of these layers in order to get it to match the surrounding paint work. While it's nominally transparent clear coat has a different refractive index to air (air is typically 1.0003, car lacquer is ~1.45) and it's this property that makes the finish look glossy or shiny.
If you just put paint on the ding with no lacquer then the painted section will look "flat" and un-reflective and even if the paint is an exact match to the existing paint on the bumper it's going to stick out like a sore thumb as a result.
So as a bare minimum if you are going to get a touch up kit you'll need to get one that includes both paint (made up to match the paint code of your car) and lacquer. Any kit that doesn't include the lacquer should be avoided like the plague!!
Also without going into the tedious optical physics of it all the refractive index basically means how much the light bends when it crosses the layer between two different materials (in this case from air to the lacquer and then from the lacquer back to the air as it reflects back off the paint) and because of this angle the thicker the layer of lacquer the further it travels at this angle and this means that if you put two sections of differing depth next to each other it looks different.
Smoothness of the surface is also a huge influence on how the end result looks - rough finished lacquer or say the transition between two layers of differing thickness (such as at the edge of a repair) mean the reflected light is going to be bouncing off all over the place which, again, sticks out like a sore thumb.
If after reading the above you still think this is going to be quicker/easier than taking it to a body shop or getting a mobile repair company out then here's the basics of how you do this:
You're going to need somewhere that's:
- Not too hot (and out of direct sunlight!)
- Well lit
- As dust free as you can manage (dust sticks to both paint and lacquer!)
- Not too humid (if the air is too "wet" many lacquers will dry cloudy!)
Stage 1 - Prep
Clean the car - you'll want to use a car shampoo that contains NO "wax" or sealant elements (so no Turtle Wax Wash'n'Wax or the like). If you don't have any then washing up liquid should be fine (it's an excellent degreaser and should strip off any wax or sealants on the car). Now you'll want to claybar the panel you are touching up. This will remove any tar, bugs or other surface contaminants. Once you've done this dry the car thoroughly with a microfibre cloth or towel and buff off any product residues.
Mask up any nearby badges, panel gaps or joins (e.g. between panels or light clusters) - you'll want a suitable low-tack masking tape, 3M 3434 is my go-to - it's cheap and effective. And 15 minutes masking up saves hours of trying to clean lacquer dust out of them later.
Stage 2 - Paint
The specific kit you buy will contain it's own instructions to help with this stage - but this is pretty much what you described in the question. You apply a very, very, fine layer of paint to the ding, and only the ding - you really want to prevent the paint getting onto the undamaged paint as much as possible (otherwise you are layering paint on top of paint + lacquer and the result will be raised from the surface of the panel). Follow the manufacturers guidelines on how long this layer needs to dry (remember keep the car away from dust, moisture etc during this time)
Stage 3 - Lacquer
Similar to the paint stage you apply the lacquer as much as possible to just the ding - although a little overlap here isn't quite as detrimental, better to overlap slightly and ensure you aren't leaving a gap. You want a very, very thin layer of the lacquer as well (if you put on too thick a coat it can be dealt with, it just means that the next stage will take a very long time). Follow the manufacturers guidelines on how long this layer needs to dry (remember keep the car away from dust, moisture etc during this time)
Stage 4 - Polish 1
Now we need to smooth the lacquer finish and get it even with the surrounding area. For this you're going to need an abrasive or "cutting" compound. Meguiars "Ultimate Compound" is my go-to here - I find it very effective but also safe. It's quite hard to accidentally damage the paintwork with it.
If you've got a dual-orbit machine polisher here this will save you a whole chunk of time but you can do this by hand it just takes ages and, well, it's exhausting. You'll want to work the ding and surrounding area until the lacquer coat is evened out. Then buff everything clean with a microfibre.
Depending on how much excess lacquer there is on the touched up area this bit can take anywhere from 30-90 mins with a machine polisher, much more if doing by hand.
Stage 5 - Polish 2
Basically repeat stage 4 - but this time with a "finishing" compound. Because you aren't looking to remove any significant amounts of clear coat here (more smoothing) this is significantly quicker than Stage 4. Once you're all buffed clean you can now remove the masking tape.
Stage 6 - Seal
Apply a generous coat of your wax or sealant of choice to protect the paintwork.
Doing minor repairs like this can be very satisfying and provide an enormous sense of accomplishment when you pull it off well - but it can be very frustrating and downright miserable when you don't. And it really isn't easy to pull off well. And it's pretty much never going to save you time and hassle - if you're looking for a low-hassle way of getting this sort of thing done as opposed to doing it for the pleasure of mastering the skill then do yourself a solid and don't.