I and others were involved with Oscar as he evolved from 15% ferric chloride to 11%, then to 7.5% for some cases (mostly depending on ambient temp -- less concentrated in warmer climes).

Coating bores to protect is fine but he relied on solid rubber stoppers when dip etching. For boiling, each bbl tube got one solid stopper and one with one hole at the other end, with a 90 degree bent stainless tube to vent the bbl interior to air above the water line.
Here are my notes on his process, made with his help:

A Rust Blue Primer
From: Oscar Gaddy

A Slow Rust Blue Primer
With regard to polishing, this should all be done by hand and never with a buffing wheel. The finest that I usually polish is 320 grit. Any finer than this is wasted effort in my opinion as the etching of the steel surface by the acidic rusting agents creates a matte finish with more surface variation than is obtained by polishing with the finer grit papers.

After polishing is completed, the barrel surfaces must be completely degreased before rusting. There are several ways that this can be done including organic solvents and the lime slurry or whiting method. I prefer to boil the barrels in a mild solution of potassium hydroxide which saponifies and removes all greases and oils. The important thing is to use a method that works for you.

After degreasing, the barrels must be handled only when wearing gloves--either rubber or cotton depending upon whether the barrels are wet or dry. The rusting solution is applied to the barrel surfaces. I use a 1 inch wide foam paint brush for this. The rusting solution should be applied as uniformly as possible but it is not extremely critical. The barrels should then be allowed to rust from 12 to 24 hours. Some people use humidity controlled chambers, but I just hang them for rusting in my unheated and un-air conditioned garage. I live in central Illinois and the humidity of the outdoor air is seldom below 30 to 40 % year round which is quite adequate for rusting. Do not try to rust indoors as the indoor humidity is usually too low especially in the winter.

After rusting, the barrels are then boiled in water to convert the brown ferric oxide to the black ferro-ferric oxide. It is always safest to use rainwater or distilled or deionized water if your water supply is very hard. Water with large amounts of some minerals have been known to convert the brown oxide to reddish colored carbonates. After boiling, the excess black oxide is then carded off. I use one of Brownells wire wheels with .005 in steel wires rotating at about 600 rpm. You can also just use steel wool and it is much easier to do this with the barrels wet under running water if you do it this way.

After carding, the rusting, boiling and carding process is repeated until the desired black finish is obtained. The exact number of passes depends upon the rusting solution and the humidity during rusting. Finally, the barrels are placed in a chemically basic solution such as potassium hydroxide or sodium bicarbonate to neutralize any acids that remain on the barrel surface or in pores in the metal in order to prevent any after-rusting. The barrels are then oiled and the bores are polished and the process is complete.
These are the basics of the process. I will be happy to try to answer any questions.

Rust Bluing Formula per Oscar Gaddy Stock Solution; dilute 1:1 … and up to 1:8 for final rustings. To make 125 ml Stock Solution:

Conc. HNO3 ..... 3.22 ml

Hg2Cl2 (Mercuric chloride -- DEADLY poison) .... 6.25 gms

C2H5OH ..... 3.25 ml

Fe2Cl3 ..... 5 ml

H2O ..... to make 125 ml

Parker Factory Process Adapted by Oscar Gaddy: (DGJ 1997 #2, #3 & 2003 #1)

1. Prep as in Oscar’s original process [next page], Steps 1 – 7

2. “Light” Etch in “mild” Fe2Cl3 solution [I used 15%, it was hard to maintain even results in hi ambient temps of FL – Oscar later moved down to ~ 10%, some report using 7.5%]

3. Rust and card once daily, for 7 to 10 days, as in Oscar’s original process, but no boiling (wet card?) [I dry carded with 0.006 wire wheel, need to check speed, 0.005 or .004 better]

4. Only after 7 to 10 passes, plug bores with rubber stoppers (one in each bore with stainless, ¼,” 90 o degree-bent tube for pressure relief), then boil 5 minutes, in solution of 3.5 ltrs water with 60 cc logwood powder + 30 cc FeSO4 . 7 H2O powder (copperas)

5. Rinse well in cold water

6. Soak ~ 5 minutes in solution of ~ 3.5 ltrs water with 15 cc FeSO4 powder (concentration not critical) – a finger should be able to rub to show light metal with contrast

7. Scrub with CaCO3 powder picked up on wet towel (paper or cloth)

8. Job should look good, but can be improved if desired by rusting and carding 2 or 3 more times; if that is done repeat final steps 4 thru 7. [Parker factory on notes 2nd following page]

The First Process Oscar Gaddy Published [which I used on Parker D bbls with good results]:

1. Metal repairs as required: Raise dents & peen upset metal

2. Strike damaged areas

3. Hand polish to 320 (up to 400 grit) finish (but more is wasted effort, matted by rusting)

4. Drill 2nd hole in bottom rib

5. Degrease (boil in mild KOH, 0.1 - 0.2 N)

6. Remove remaining finish with Naval Jelly [or Brownell’s version for max polish]

7. Scrub with Ajax, rinse thoroughly and dry

8. Rubber stopper and rust with 1:1 dilution of rusting formula listed above; apply with 1 inch foam paintbrush with excess squeezed out; [make & use rib hole plugs next time]

9. Let stand and rust for 24 hours [but check progress, perhaps shorter time]

10. Boil in distilled H2O for several minutes; dry thoroughly

11. Dry card with 6” diameter wire wheel, 600 RPM, wires 0.006” – minimum pressure against wheel, (consider 0.005” for less aggressive removal or try hand carding next time)

12. Plug with rubber stoppers; immerse in Fe2Cl3 – commercial solution (Radio Shack) diluted to15% strength with H2O at room temperature for “a few seconds” – dilute even more, to 10%, 7.5% or perhaps even 5% at higher ambient temperatures -- [ I later learned that one, longer etch only, before bluing, is OK – see Parker factory process – still ambiguous on this]

13. Quickly rinse in H2O and wet card vigorously with grade 0 steel wool until no more finish can be removed – if not diluting to or below 10% consider dipping into a PVC pipe water tank rinse [rather than hose down] to even out etch time breech to muzzle

14. Repeat steps above as indicated, 10 to 15 times, diluting rusting solution after 2 to 3 passes – typically finishing up with 1:8 dilution for the final rustings – perhaps even lower

15. Immerse in dilute KOH (0.1 - 0.2N) for ~ 1 hour [I repeatedly injected KOH solution into inter-barrel region and sloshed it around], then …

16. Rinse thoroughly, then …

17. Dry, oil, RIG or wax thoroughly, and …

18. Polish bores, muzzle, lump and breech faces

Parker Factory Process (Unmodified, Verbatim from Parker notes – Oscar, 2003)

1. Clean with brush, heat “on steam pipes” until “fairly hot”

2. Wash with H2SO4, diluted 1:2, for 4 or 5 minutes to bring out the figure

3. Wash with water to kill the acid

4. Clean with pumice and water

5. Apply 1st rusting acid: ½ oz HCl + ½ oz CuSO4 in 1 qt water

6. Apply 2nd rusting acid while first is still wet; formula varied for weather:

Dry weather: ½ oz HCl + ½ oz HgCl + ¼ oz Fe2Cl3 in 1 qt water

Damp weather: ¼ oz HCl + ½ oz HgCl in 1 qt water

7. Place in steam dry[er?] for ~ 5 minutes

8. Put over to the side of the room after drying [to rust?]

9. Turn off “wet steam” after rusted and turn on “dry steam” to avoid too much rust

10. Let stand over night

11. Card with wire brush

12. When cold [implies prior step used power] repeat rusting exactly as before, 4 to 6 times

13. Cook in logwood bath 4 to 5 minutes; bath = 2 qts logwood [probably chips] + 4 to 6 ozs copperas 1.5 ozs CuSO4 in “tank” of water

14. Cool off and place in copperas bath; bath = ½ oz copperas in “tank” of water until finger can rub off black coating to show white, usually ~ 5 minutes

15. “Wash [implies rub] with whiting [CaSO4 fine powder] until figure shows even”

16. Wash under running water

17. Put into tank of hot water, wash well

18. Dry in sawdust

19. Rust again in two acids as before (Steps 5 & 6)

20. Cook in steam room [one time]

21. Cook in logwood + copperas bath 1 to 3 minutes

22. Put into copperas bath, finish as before