No. 90-1420. Argued March 21, 1991. Decided July 16, 1991. Fred M. Plevin, with whom James K. Smith was on the brief, San Diego, Cal., for petitioner. Frederick C. Havard, Atty., N.L.R.B., with whom Jerry M. Hunter, Gen. Counsel, and Aileen A. Armstrong, Deputy Associate Gen. Counsel, were on the brief, Washington, D.C., for respondent. Robert A. Cantore, with whom Steven M. Rehaut, Los Angeles, Cal., for Sheet Metal Workers Trust Fund, and Richard D. Prochazka, San Diego, Cal., for Sheet Metal Workers'