Wet-sanding - Fresh Paint vs Factory Paint
Fresh paint sprayed at a local body shop
Fresh paint will tend to be softer
Modern basecoat/clearcoat paints are catalyzed, that is they are chemically hardened, a simple analogy would be the way you mix a 2-part epoxy glue together and the resulting product hardens through a chemical reaction. If you've ever worked with fiberglass and polyester resin where a small amount of catalyst, (hardener), is added to the resin and the chemical reaction cause the resin to cure and harden, that's another simple analogy as to how catalyzed basecoat/clearcoat paints are hardened.
Contrast this to older style paints which were called solvent-evaporation paints in which the paints dried without the use of a catalyzing agent and instead simply dried and hardened over a longer period of time as the solvents mixed into the paint evaporated.
Even though most modern paints are chemically cured, there is still a window of time where these paint are not 100% hardened and during this
window-of-time the paint is what is sometimes refereed to as still wet, not like in
wet gooey paint but as in still soft enough to easily sand and buff.
There are also what are called fast drying, medium drying and slow drying reducers which are solvents for mixing in with the paint to give it a thinner viscosity before spraying. A painter can match their choice of reducer to speed up or slow down the drying time.
All these factors can affect paint hardness or softness in the first few days that follow after the car leaves the paint booth.
Usually, if the painters knows the car is to be wet-sanded after painting he can adjust how he mixes the paint to give the person usually called "The Painter's Helper" time to sand and buff the paint before it becomes too hard. Time of year which includes temperature and humidity can also play huge factors in drying or curing time and can be factored in and adjusted for.
Now here's what's key about this,
sanding paint is always easy whether the paint is fresh or baked-on at the factory, in simple terms, (very simple terms for the purpose of explanation), sanding paint is putting scratches
into the paint.
Again... sanding paint is easy, it's the part where you try to remove your sanding marks that can be difficult. The harder the paint the more difficult it will be to buff out your sanding marks. The softer the paint the easier it will be to remove your sanding marks.
Fresh paint will tend to be thicker
At the body shop level, again if the painter knows the car is to be sanded and buffed, they'll usually add an extra coat or more of paint, this gives the painter's helper a little more wiggle room to sand and buff and not worry about sanding or buffing through the clear coat and exposing the basecoat.
I've also met painters that will just spray a thicker or heavier coat and only spray 2 coats of paint but through factors they can control they can spray it on thicker and therefore not have to spray a third or fourth coat.
Fresh paint will be sprayed on custom projects with more planning and higher expectations.
If you're having a custom car project painted, for example you spent months and more than likely years rebuilding a classic Mustang and now it's time to get it painted, in most cases you'll be meeting with the painter ahead of time planning out the paint job and it's at this time you discuss with them your expectations. If you want the car sanded flat and then buffed to a high gloss for a true show car finish, (if this is you goal or expectation), then the painter will spray an extra coat or two of paint to provide plenty of film-build for the painter's helper to safely sand the paint flat. They will also build in the cost of the extra
materials, (clear paint),
time and
labor to your bill.
Summery
Fresh paint sprayed at your local body shop, specifically the clear layer, will tend to be thicker than the clear layer that comes from the factory and it will tend to be softer and easier to sand and buff shortly after the car comes out of the paint booth. Because it's thicker there is a little more safety margin or wiggle-room for the person to sand and buff the paint and not break-through the clear layer and expose the basecoat. Because the paint is fresh it's going to be softer than factory baked-on paint, at least for a window of time and this will make buffing out the sanding marks faster and easier.
Factory baked-on paint
Factory paint will be harder
The original paint sprayed onto your car as it traveled down the assembly line at the manufacturer's plant is in most cases baked-on at high temperatures before any wiring or the interior is installed and for this reason higher temperatures can be used since there's nothing to melt of catch on fire in or on the car yet. By the time the car pops-out the end of the assembly line the paint is fully cured and hardened. For this reason it will still be easy to sand, (that's putting scratches into the paint), but it will be more difficult to remove your sanding marks out of the paint.
Factory paint will be thinner
At the factory, the paining process is very automated and the amount of clear paint applied to the vehicle is done so in a tightly controlled manner and to very stringent specifications. You don't have the ability to ask for an extra coat of clear, or for a thicker coat of paint to be sprayed and from a materials cost point of view, it's probably safe to say that the amount of paint used to coat each car trends towards being the minimum amount, not a generous amount. Simply put, factory clear coat paints tend to be very thin compared to what you can get at your local body shop.
Summary
Factory paint will tend to be hard and thin, it will be easy to sand but more difficult to remove your sanding marks out 100%
Factory paint will be thin and if you're not really careful you'll break-through the clear layer and expose the basecoat or color coat either during the sanding process or the ensuing buffing process as both procedures remove a little paint.
