Let me see if I cant help with this issue, as I have a bit of a background in finishes.
Gloss levels are available in a a wide range of gloss levels. The ranges of gloss are developed by the ASTM and NPCA as standards.
Flat has a gloss range below 3 at a 60 deg meter
Eggshell hass a gloss range between 5 to 10 at a 60 deg meter
Eggshell/Satin/Pearl depending on the brand is 10 to 20 gloss range at a 60 deg meter
Semi-Gloss is 35 to 70 gloss range at a 60deg meter.
Gloss is over 70 gloss range at a 60 deg meter.
To the visual eye there are factors that also effect the gloss, with lighting being the most common factor. Ever notice how a color on a wall seems different depending on the lighting such as lamp lighting, sunlight, the coolness of the bulb, morning or evening light.
What one company calles Eggshell another may call satin. Some Semi glosses run closer 60 degree gloss range and another company may list their semi gloss at closer to 45.
So you have to have a good base as to what is acceptable to you for frame black.
As to preferred finishes for undercarriages, K frames, and other parts I suggest the following options.
I prefer a powder coating personally. Most powder coat mfg such as PPG and others have more than 12 blacks with various sheens. I personally recommend something in a 40-60 gloss range.
1. Dont use a spray can product line. While some are better than others they dont offer the millage needed. Sooner or later you will have a thin spot that salt, and corrosion will find a way thru and spread. This will happen, no doubt about it.
2. Epoxies are a tough 2 component finish with outstanding hardness, abrasion resistance, alkali and acid resistance. Its adhesion is very good and finish is smooth. Its easy to clean and lasts for years under even sever condidtions.
3. Aliphaltic Urethanes are also 2 component finishes and also provide superior chemical and stain resistance, "plus have superior gloss and color retention" even above epoxies. It will have more of a tile like finish. Another advantage is that Urethanes will dry to handle or touch faster than any other HD topcoat. PPG makes a High Build Semi-Gloss Urethane called Pitthane. 95-8300 series that you will find in architectural dealers and stores. It comes with a short filled gallon A component and a pint of B. You can recoat it in 2 hours. Some of us thin the first coat about 5% with thinner or acetone and then full strength in a 2nd coat. It can be sprayed with a .013 to .015 tip when using an airless or if your using a conventional spray Devilbiss gun a 704 or 777 air cap with E tip or needle.
On another note. PPG makes a Urethane called Durethane DTM (direct to metal) this stuff goes on metal without priming or really much cleaning. While its a little too glossy I think for frames, its great for spraying metal when you dont want to prepare it much at all or you dont want to spend time and labor priming, letting it dry and coming back with a topcoat. Bang! just spray it on metal that even has tight adhering rust. Runs $75 to $90 a gallon.
Dont use any latex or Alkyds (oil) on automotive applications. One thing, they cant stand the heat, it will yellow and in time chalk or fade.
Urethanes and Epoxies or used in chemical plant on tanks that have caustic, high acidity, and areas that have heavy foot traffic and frequently cleaned with caustic chemicals.
