Slow Function

Any suggestions to speed up this funtction? 350 Rows takes over 1 minute. Maybe using stuff?

CREATE function GetStockDesc(@StockNumber varchar(8000))
RETURNS varchar(8000) AS     
BEGIN    
DECLARE @StockDesc varchar(8000)
set  @StockDesc = ''

 
Select  @StockDesc = isnull(rtrim(ltrim(@StockDesc)) + ', ', '') + brandcat.desc1
From    brandcat
Where  brandcat.number  =  @StockNumber


if(LEN(@StockDesc) > 0)
set @StockDesc  = substring(LTRIM(rtrim(@StockDesc)), 2,LEN(@StockDesc))  

 

  
  
RETURN  @StockDesc    
END

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