Across:
   1. see 6 down
   4,27. Originally, traditional eggy medium (paint) 6 rudely applied over novel canvas (a really drab backing) of anguish's radical depiction? (7 2 9)
   9. Short film finishes after inspector returns (9)
  10. Some strode to victory with backing as stated preference in poll (5)
  11. Wary on straying into home of 6? (6)
  12. Messes up with stupid arrangement involving sister (8)
  14. A deal is cut, adapted, and made real (10)
  16. Egyptian cobras and mambas psychically possessed (4)
  18. One getting married is very impressed (4)
  19. Ire is about muddled biographies of the deceased (10)
  22. Local is confused by end of day, along with other people (8)
  23. Institution where some of them use umbrellas (6)
  26. Requirements regularly anger eldest (5)
  27. see 4 across
  28. California mother lies about "tea" plants (9)
  29. Gate, perhaps, when trying to restrict (5)
Down:
   1. Nomad, an original 1 across, going walkabout with 15 (7)
   2. How often you want to repeat traumatic experience, if nerve lost? (5)
   3. Hard place to find hard concept and method (8)
   4. Note unusual feeling (4)
   5. Young lady has blown credit points unfortunately (10)
   6,1ac. He painted 15 with strangely curved hand around mouth, opening (6 5)
   7. Obsidian tip, as tidier, providing nice little things to 1 across (9)
   8. They don't like being dressed in studs, abused (7)
  13. Horrible acid, all bio-engineered (10)
  15. Art scheme ruined as 4 across 27 pinched from 1 across 23 (3 6)
  17. Dear composition of blue lava (8)
  18. Unhealthy stuff in coarse nicotine patch (7)
  20. Dose may change at uncertain time (7)
  21. Demand made for 15, or man's corruption (6)
  24. Very particular sex acts offered (5)
  25. Rams injured limbs (4)